Science and Technology: previous-year questions
185 real UPSC Prelims questions (2011–2026) with the official answer and a clear explanation. 50 of them came in the last five papers.
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What are the reasons for the people’s resistance to the introduction of Bt brinjal in India?
- 1. Bt brinjal has been created by inserting a gene from a soil fungus into its genome.
- 2. The seeds of Bt brinjal are terminator seeds and therefore, the farmers have to buy the seeds before every season from the seed companies.
- 3. There is an apprehension that the consumption of Bt brinjal may have adverse impact on health.
- 4. There is some concern that the introduction of Bt brinjal may have adverse effect on the biodiversity.
Select the correct answer using the codes given below :
Answer (c) & explanation
Statements 3 and 4 are correct (c). The 2010 moratorium on Bt brinjal followed public fears about possible health effects of eating it (3), and concern that it could contaminate India's many native brinjal varieties, since India is a centre of origin of brinjal (4). Statement 1 is wrong: the Bt gene comes from a bacterium, Bacillus thuringiensis, not a fungus. Statement 2 is wrong: Bt brinjal seeds are not terminator seeds; terminator technology was never commercialised, and public-sector Bt brinjal varieties could be saved. So (a), (b) and (d) fail.
Notice the context: GEAC cleared it in 2009, but Jairam Ramesh imposed the moratorium after public hearings in February 2010. Bangladesh has grown Bt brinjal since 2014. Trap: two plausible myths are mixed with two real concerns.
Answer as per the official UPSC answer key.
Other than resistance to pests, what are the prospects for which genetically engineered plants have been created?
- 1. To enable them to withstand drought
- 2. To increase the nutritive value of the produce
- 3. To enable them to grow and do photosynthesis in spaceships and space stations
- 4. To increase their shelf life
Select the correct answer using the codes given below :
Answer (c) & explanation
Items 1, 2 and 4 are correct (c). Genetic engineering has produced crops for drought tolerance, such as Argentina's HB4 wheat and drought-tolerant maize (1), better nutrition, such as Golden Rice with beta-carotene, approved in the Philippines (2), and longer shelf life, such as the Flavr Savr tomato (1994) and non-browning Arctic apples (4). Item 3 is wrong: there are no GM plants created to photosynthesise in spaceships; plants are grown in space experiments, but not engineered for that purpose. So (a), (b) and (d) fail.
Notice India's position: Bt cotton remains the only GM crop grown commercially; GM mustard DMH-11 got environmental release approval in 2022 but is in court, and genome-edited rice varieties released in May 2025 are not classed as GM. Trap: space farming exists, but not as a GM aim.
Answer as per the official UPSC answer key.
Consider the following kinds of organisms :
- 1. Bacteria
- 2. Fungi
- 3. Flowering plants
Some species of which of the above kinds of organisms are employed as biopesticides?
Answer (d) & explanation
All three are correct (d). Biopesticides come from living organisms of all three kinds. Bacteria: Bacillus thuringiensis (Bt) makes proteins toxic to caterpillars (1). Fungi: Trichoderma controls soil-borne diseases, and Beauveria and Metarhizium infect insects (2). Flowering plants: neem (azadirachtin) and pyrethrum from chrysanthemum flowers act as natural insecticides (3). So options (a), (b) and (c) leave out real sources.
Notice the advantages: biopesticides are specific, biodegradable and safer for pollinators. Viruses such as NPV (nuclear polyhedrosis virus) are also used. In India they are registered under the Insecticides Act, 1968, and promoted under the National Mission on Natural Farming. Trap: plants are often forgotten as a source.
Answer as per the official UPSC answer key.
Recently, there has been a concern over the short supply of a group of elements called ‘rare earth metals’. Why?
- 1. China, which is the largest producer of these elements, has imposed some restrictions on their export.
- 2. Other than China, Australia, Canada and Chile, these elements are not found in any country.
- 3. Rare earth metals are essential for the manufacture of various kinds of electronic items and there is a growing demand for these elements.
Which of the statements given above is/are correct?
Answer (c) & explanation
Statements 1 and 3 are correct (c). In 2010, China, which then produced over 90% of the world's rare earths, cut export quotas (1), just as demand was rising for magnets, phones, batteries, wind turbines and electric vehicles (3). Statement 2 is wrong: rare earths are found in many countries, including India (monazite sands), the USA, Brazil, Vietnam and Russia. So (a), (b) and (d) fail.
Notice the recent turn: China imposed export controls on several rare earths and magnets in April 2025 and widened them in October 2025, hurting automakers. India launched a National Critical Mineral Mission (2025) and a scheme for rare earth permanent magnets (November 2025); IREL handles domestic output. Trap: 'rare' refers to concentration, not scarcity.
Answer as per the official UPSC answer key.
Biomass gasification is considered to be one of the sustainable solutions to the power crisis in India. In this context, which of the following statements is/are correct?
- 1. Coconut shells, groundnut shells and rice husk can be used in biomass gasification.
- 2. The combustible gases generated from biomass gasification consist of hydrogen and carbon dioxide only.
- 3. The combustible gases generated from biomass gasification can be used for direct heat generation but not in internal combustion engines.
Select the correct answer using the codes given below :
Answer (a) & explanation
Statement 1 only is correct (a). A gasifier partly burns biomass in limited air to make producer gas. Coconut shells, groundnut shells and rice husk are all good feedstocks (1). Statement 2 is wrong: the gas is mainly carbon monoxide, hydrogen and methane, with nitrogen and some CO₂; carbon dioxide does not burn. Statement 3 is wrong: the gas can be used for heat and also to run internal combustion engines for electricity, as in village power plants. So (b), (c) and (d) fail.
Notice the example: Husk Power Systems in Bihar has lit villages using rice-husk gasifiers since 2007. India's SATAT scheme promotes compressed biogas, a different, biological route. Trap: 'only' and 'but not' make statements 2 and 3 false.
Answer as per the official UPSC answer key.
Consider the following statements :
If there were no phenomenon of capillarity
- 1. it would be difficult to use a kerosene lamp
- 2. one would not be able to use a straw to consume a soft drink
- 3. the blotting paper would fail to function
- 4. the big trees that we see around would not have grown on the Earth
Which of the statements given above are correct?
Answer (b) & explanation
Statements 1, 3 and 4 are correct (b). Capillarity draws liquid up narrow spaces because of adhesion and surface tension. It lifts kerosene up a lamp wick (1), soaks ink into blotting paper (3), and helps raise water up the narrow xylem vessels of trees, along with transpiration pull and cohesion (4). Statement 2 is wrong: drinking through a straw works by air pressure; sucking lowers the pressure inside, and atmospheric pressure pushes the drink up. So (a), (c) and (d) fail.
Notice the related examples: water rising in soil and in a towel, and the meniscus in thin tubes. Mercury shows capillary fall, not rise, because cohesion exceeds adhesion. Trap: the straw is about pressure, not capillarity.
Answer as per the official UPSC answer key.
What is the role of ultraviolet (UV) radiation in the water purification systems?
- 1. It inactivates/kills the harmful microorganisms in water.
- 2. It removes all the undesirable odours from the water.
- 3. It quickens the sedimentation of solid particles, removes turbidity and improves the clarity of water.
Which of the statements given above is/are correct?
Answer (a) & explanation
Statement 1 only is correct (a). Ultraviolet light, especially at about 254 nm, damages the DNA of bacteria and viruses, so they cannot multiply; this inactivates harmful microbes (1). Statement 2 is wrong: UV adds no chemicals and does not remove odours; activated carbon filters do that. Statement 3 is wrong: UV does not settle particles or remove turbidity; in fact, cloudy water shields microbes from UV, so water must be filtered first. So (b), (c) and (d) fail.
Notice the related methods: chlorination leaves a residual disinfectant, reverse osmosis removes dissolved salts, and ozonation also disinfects. UV does not remove arsenic or fluoride. Trap: UV is a disinfection step only.
Answer as per the official UPSC answer key.
Graphene is frequently in news recently. What is its importance?
- 1. It is a two-dimensional material and has good electrical conductivity.
- 2. It is one of the thinnest but strongest materials tested so far.
- 3. It is entirely made of silicon and has high optical transparency.
- 4. It can be used as ‘conducting electrodes’ required for touch screens, LCDs and organic LEDs.
Which of the statements given above are correct?
Answer (c) & explanation
Statements 1, 2 and 4 are correct (c). Graphene is a single layer of carbon atoms arranged in a honeycomb, so it is two-dimensional and conducts electricity very well (1). It is one atom thick yet about 200 times stronger than steel (2). Being nearly transparent and conductive, it suits conducting electrodes in touch screens, LCDs and OLEDs (4). Statement 3 is wrong: it is made of carbon, not silicon. So (a), (b) and (d) fail.
Notice the history: Andre Geim and Konstantin Novoselov won the 2010 Nobel Prize in Physics for isolating it with sticky tape. India set up the India Graphene Engineering and Innovation Centre in Kerala. Trap: 'silicon' comes from the link with electronics.
Answer as per the official UPSC answer key.
What is the difference between a CFL and an LED lamp ?
- 1. To produce light, a CFL uses mercury vapour and phosphor while an LED lamp uses semi-conductor material.
- 2. The average life span of a CFL is much longer than that of an LED lamp.
- 3. A CFL is less energy-efficient as compared to an LED lamp.
Which of the statements given above is/are correct ?
Answer (c) & explanation
Statements 1 and 3 are correct (c). A CFL passes current through mercury vapour, whose ultraviolet light makes a phosphor coating glow, while an LED emits light from a semiconductor chip (1). LEDs give more light per watt, so a CFL is less energy-efficient (3). Statement 2 is reversed: an LED lasts much longer, about 25,000 hours or more, against roughly 8,000 for a CFL. So (a), (b) and (d) fail.
Notice the policy: India's UJALA scheme (2015) distributed over 36 crore LED bulbs, cutting their price sharply, and the Street Lighting National Programme replaced street lamps with LEDs. CFLs also pose a mercury disposal problem. The 2014 Nobel Prize in Physics went for the blue LED.
Answer as per the official UPSC answer key.
Satellites used for telecommunication relay are kept in a geostationary orbit. A satellite is said to be in such an orbit when :
- 1. The orbit is geosynchronous.
- 2. The orbit is circular.
- 3. The orbit lies in the plane of the Earth’s equator.
- 4. The orbit is at an altitude of 22,236 km.
Select the correct answer using the codes given below :
Answer (a) & explanation
Statements 1, 2 and 3 are correct (a). A geostationary orbit is a special geosynchronous orbit: the satellite completes one revolution in one sidereal day (about 23 h 56 min) (1). It must be circular (2) and lie in the plane of the equator (3), moving west to east, so it appears fixed above one spot. Statement 4 is wrong: the altitude is about 35,786 km, which is 22,236 miles, not 22,236 km. So options (b), (c) and (d) fail.
Notice the difference: every geostationary orbit is geosynchronous, but a geosynchronous orbit can be inclined or elliptical and then traces a figure-of-eight. India's GSAT and INSAT satellites use GEO, launched on the GSLV and LVM3. The NavIC system uses both geostationary and inclined geosynchronous satellites. Trap: the number 22,236 is right only in miles.
Answer as per the official UPSC answer key.
A married couple adopted a male child. A few years later, twin boys were born to them. The blood group of the couple is AB positive and O negative. The blood group of the three sons is A positive, B positive, and O positive. The blood group of the adopted son is
Answer (a) & explanation
O positive is correct (a). An AB parent passes either A or B, and an O parent passes only O. So the couple's own children can only be A (genotype AO) or B (genotype BO); an O child is impossible. The twins must be the A positive and B positive sons, and the O positive son must be adopted. (b) and (c) are the twins' groups. (d) is wrong: the data are enough to decide.
Notice the Rh factor: the O parent is Rh negative, the AB parent positive, so Rh-positive children are possible and do not change the answer. Another rule: AB is the universal recipient and O negative the universal donor. Trap: the Rh signs are a distraction.
Answer as per the official UPSC answer key.
At present, scientists can determine the arrangement or relative positions of genes or DNA sequences on a chromosome. How does this knowledge benefit us ?
- 1. It is possible to know the pedigree of livestock.
- 2. It is possible to understand the causes of all human diseases.
- 3. It is possible to develop disease-resistant animal breeds.
Which of the statements given above is/are correct ?
Answer (c) & explanation
Statements 1 and 3 are correct (c). Gene mapping finds the positions of genes and DNA markers on chromosomes. This helps confirm the pedigree or parentage of livestock through DNA markers (1). It also helps breeders identify genes for traits such as disease resistance and use marker-assisted selection to develop resistant breeds (3). Statement 2 is wrong because of the word 'all': many diseases come from germs, diet, environment or injury, so gene maps cannot explain every human disease. So options (a), (b) and (d) fail.
Notice the bigger picture: the Human Genome Project finished in 2003, and India's Genome India Project released data from 10,000 genomes in 2025. Genome mapping also supports gene therapy, such as India's first CRISPR-based sickle cell research, and precision medicine. Trap: absolute words like 'all' make a statement suspect.
Answer as per the official UPSC answer key.
A new optical disc format known as the Blu-ray Disc (BD) is becoming popular. In what way is it different from the traditional DVD ?
- 1. DVD supports Standard Definition video while BD supports High Definition video.
- 2. Compared to a DVD, the BD format has several times more storage capacity.
- 3. Thickness of BD is 2.4 mm while that of DVD is 1.2 mm.
Which of the statements given above is/are correct ?
Answer (b) & explanation
Statements 1 and 2 are correct (b). A Blu-ray disc is read by a blue-violet laser (405 nm) with a shorter wavelength than a DVD's red laser, so data packs more tightly: about 25 GB per layer against 4.7 GB on a DVD. That gives several times more storage (2) and room for High Definition video, while DVD is mainly standard definition (1). Statement 3 is wrong: both discs are 1.2 mm thick; Blu-ray's data layer just sits nearer the surface. So (a), (c) and (d) fail.
Notice the trend: Ultra HD Blu-ray stores up to 100 GB for 4K video, but streaming has largely replaced discs. Trap: '2.4 mm' doubles the real thickness.
Answer as per the official UPSC answer key.
Microbial fuel cells are considered a source of sustainable energy. Why ?
- 1. They use living organisms as catalysts to generate electricity from certain substrates.
- 2. They use a variety of inorganic materials as substrates.
- 3. They can be installed in waste water treatment plants to cleanse water and produce electricity.
Which of the statements given above is/are correct ?
Answer (c) & explanation
Statements 1 and 3 are correct (c). A microbial fuel cell uses living bacteria as catalysts to break down substrates and release electrons, which flow through a circuit as electric current (1). It can be fitted in wastewater treatment plants, where bacteria feed on organic matter in sewage, cleaning the water while generating electricity (3). Statement 2 is wrong: the substrates are mainly organic materials such as glucose, acetate and sewage waste, not a variety of inorganic materials. So options (a), (b) and (d) fail.
Notice why it is called sustainable: it turns waste into power, works at normal temperatures, and does not burn fuel. Related ideas include biogas from anaerobic digestion and bio-electrochemical systems. India's SATAT and GOBARdhan schemes also aim at waste-to-energy. Trap: 'living organisms' in statement 1 is correct, since bacteria are the catalysts, not an error.
Answer as per the official UPSC answer key.
An artificial satellite orbiting around the Earth does not fall down. This is so because the attraction of Earth
Answer (d) & explanation
Option (d) is correct. A satellite is always 'falling' towards Earth, but it moves sideways so fast that it keeps missing it. Earth's gravity supplies the centripetal acceleration that bends its straight path into a circle. (a) is wrong: gravity certainly exists in orbit; at the ISS height it is still about 90% of surface gravity. (b) is wrong: the Moon's pull is tiny and does not cancel Earth's. (c) is wrong: gravity provides acceleration towards the centre, not the forward speed; the speed comes from the launch.
Notice the numbers: a low Earth orbit needs about 7.8 km/s; a geostationary satellite at about 35,786 km circles once in 24 hours. Astronauts float because they are in free fall, not because gravity is absent.
Answer as per the official UPSC answer key.
When the bark of a tree is removed in a circular fashion all around near its base, it gradually dries up and dies because
Answer (b) & explanation
The answer is (b). Removing a ring of bark all round the trunk is called girdling or ringing. The bark contains the phloem, which carries sugars made in the leaves down to the roots. Once the phloem is cut, the roots no longer get food, so they gradually starve of energy, stop absorbing water and minerals, and the tree dries up and dies. Option (a) is wrong because water rises through the xylem in the inner wood, which girdling leaves intact, so the leaves stay green for some time. Options (c) and (d) are not the main reason.
Notice the classic experiment: in girdling, the bark just above the ring swells, because food carried down by the phloem piles up there. This proves the phloem carries food downwards. Xylem carries water up, mainly pulled by transpiration. Trap: water still reaches the leaves; it is the roots that starve.
Answer as per the official UPSC answer key.
What is the difference between asteroids and comets ?
- 1. Asteroids are small rocky planetoids, while comets are formed of frozen gases held together by rocky and metallic material.
- 2. Asteroids are found mostly between the orbits of Jupiter and Mars, while comets are found mostly between Venus and Mercury.
- 3. Comets show a perceptible glowing tail, while asteroids do not.
Which of the statements given above is/are correct ?
Answer (b) & explanation
Statements 1 and 3 are correct (b). Asteroids are small rocky or metallic bodies, while comets are 'dirty snowballs' of frozen gases, ice and dust held with rock (1). Near the Sun, a comet's ice vaporises and forms a glowing coma and tail pointing away from the Sun, while asteroids show no tail (3). Statement 2 is wrong: asteroids are mostly in the belt between Mars and Jupiter, but comets come from the distant Kuiper Belt and Oort Cloud, not between Venus and Mercury. So (a), (c) and (d) fail.
Notice the missions: NASA's DART (2022) changed an asteroid's orbit, OSIRIS-REx returned Bennu samples (2023), and the interstellar comet 3I/ATLAS was seen in 2025. Trap: statement 2 puts comets in the inner solar system.
Answer as per the official UPSC answer key.
A genetically engineered form of brinjal, known as the Bt-brinjal, has been developed. The objective of this is
Answer (a) & explanation
To make it pest-resistant is correct (a). Bt brinjal carries the cry1Ac gene from the soil bacterium Bacillus thuringiensis. The gene makes a protein toxic to the fruit and shoot borer, the main pest of brinjal, so farmers need fewer insecticide sprays. (b) is wrong: the change does not target taste or nutrition. (c) is wrong: drought tolerance needs other genes. (d) is wrong: longer shelf life was the aim of the 'Flavr Savr' tomato, not Bt brinjal.
Notice the policy story: GEAC cleared Bt brinjal in 2009, but the environment ministry imposed a moratorium in February 2010, which still holds; Bangladesh grows it. Bt cotton (2002) remains India's only commercial GM crop, while GM mustard DMH-11 awaits final clearance.
Answer as per the official UPSC answer key.
Regular intake of fresh fruits and vegetables is recommended in the diet since they are a good source of antioxidants. How do antioxidants help a person maintain health and promote longevity ?
Answer (c) & explanation
They neutralise free radicals is correct (c). Normal metabolism, pollution and smoking produce free radicals, unstable molecules with an unpaired electron that damage cell membranes, proteins and DNA, and are linked to ageing, heart disease and cancer. Antioxidants such as vitamins C and E, beta-carotene and plant polyphenols donate electrons and neutralise them. (a) is wrong: antioxidants do not activate enzymes for vitamin synthesis. (b) is wrong: oxidation of food is how we get energy; it is not wasteful. (d) is wrong: they are not mainly gene activators.
Notice the related idea: this damage is called oxidative stress. Large trials show antioxidant supplements give little benefit, while whole fruits and vegetables do. Trap: (b) misreads 'oxidation' as bad in general.
Answer as per the official UPSC answer key.
The function of heavy water in a nuclear reactor is to
Answer (a) & explanation
Slowing down neutrons is correct (a). Heavy water (D₂O) is a moderator. Fast neutrons released in fission are too quick to be captured by uranium-235; colliding with deuterium nuclei slows them to thermal speed so they cause further fission and sustain the chain reaction. Heavy water absorbs few neutrons, so it lets reactors run on natural uranium. (b) is the opposite. (c) is incomplete: heavy water also serves as coolant in India's PHWRs, but its defining function asked here is moderation. (d) is wrong: control rods of boron or cadmium absorb neutrons to slow or stop the reaction.
Notice India's fleet: most Indian reactors are Pressurised Heavy Water Reactors, matching stage one of the three-stage programme.
Answer as per the official UPSC answer key.