In Rajasthan, which of the following districts are badly affected by the ecological problem of 'SEM'?

The correct answer is C, Water Conservation. Tarun Bharat Sangh works on water conservation, and its base is at Bhikampura in Alwar district of Rajasthan. Led by Rajendra Singh, known as the waterman of India, villagers of the area built and repaired thousands of johads, the traditional earthen check dams that hold rainwater and let it sink into the ground. Wells filled again, fields came back under crops and streams such as the Arvari, dry for years, began to flow, which is why the body is studied as a model of community-led water harvesting. Rajendra Singh received the Ramon Magsaysay Award in 2001 and the Stockholm Water Prize in 2015. A is wrong because women's groups are part of the work but not its field. B is wrong because fighting corruption is not its subject. D is wrong because it is not a tribal youth organisation, and Alwar is not a tribal belt. Exam tip: Tarun Bharat Sangh, Rajendra Singh, johads of Alwar and the revived Arvari river.

The correct answer is B, Earthworm. The earthworm is called the farmer's friend because its work keeps soil fit for crops. As it burrows it loosens the soil and lets air and water reach the roots, and the castings it leaves behind are rich in nitrogen, phosphorus and potash, so the topsoil is steadily renewed. Charles Darwin called earthworms nature's ploughmen for this reason. Farmers use the same habit in vermicomposting, where species such as Eisenia fetida turn cattle dung and crop waste into fine manure within a few weeks. A is wrong because the cockroach is a household pest that spoils food and carries germs. C is wrong because the honey bee is useful as a pollinator and for honey, but the name friend of the farmer is not given to it. D is wrong because the housefly spreads diseases such as typhoid and cholera. Exam tip: earthworm, the farmer's friend and nature's plough, used in vermicompost.

The correct answer is D, Gaur. The gaur, also called the Indian bison, is the wild bovine closest to domestic cattle and is protected in the Nagarhole and Bandipur national parks of Karnataka. It is the largest wild cattle species in the world, standing up to about two metres at the shoulder, with a high muscular ridge on its back and white stockings on its legs. It grazes and browses in moist deciduous forest and hill country, and is listed as Vulnerable on the IUCN Red List and in Schedule I of the Wild Life Protection Act of 1972. A is wrong because Red Angus is a domestic breed of beef cattle from Scotland, not a wild animal. B is wrong because the Asiatic wild buffalo is a separate species found mainly in Assam, Chhattisgarh and Odisha. C is wrong because the nilgai is an antelope, not a member of the cattle group. Exam tip: remember the pair - gaur is the Indian bison of Nagarhole and Bandipur, and the mithun is its domesticated form in the north-east.

The correct answer is D, aquifer. An aquifer is a body of rock or loose sediment that is both saturated with groundwater and permeable enough to let that water move freely, so a well sunk into it keeps yielding water. Sand, gravel and fractured sandstone or limestone make good aquifers; clay holds water but will not release it at a useful rate, and such a layer is called an aquiclude. An unconfined aquifer is open to the surface and is recharged directly by rain, while a confined aquifer is sealed between impermeable layers and can be under enough pressure to give an artesian well. Option A is wrong because karst is the cave-and-sinkhole landscape produced where limestone dissolves, not the water-bearing unit itself. Option B is wrong because an estuary is the mouth of a river where fresh water meets the sea. Option C is wrong because a reservoir stores water on the surface behind a dam. Exam tip: an aquifer yields water, an aquiclude holds it back.

The correct answer is A, Unreliable supply. Most renewable sources depend on the weather, so the power they give is intermittent rather than steady. Solar panels make nothing at night and little under thick cloud, wind turbines stop when the wind drops, and hydropower falls away in a weak monsoon. Because electricity has to match demand minute by minute, this variability is the main engineering problem with renewables, and it is why grids need storage, backup plants or a wide mix of sources. Option B is wrong because costly waste disposal is a drawback of nuclear power, not of solar or wind. Option C is wrong because renewables are valued precisely for releasing almost no carbon dioxide or particulate pollution while they run. Option D is wrong because their running cost is very low, since sunlight and wind are free and the big expense is the one-time cost of installation. Exam tip: renewables have a high capital cost and an unreliable supply, but a low running cost and low pollution.