Which of the following Sustainable Development Goals (SDG) seeks to 'Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss'?

The correct answer is C, Carbon monoxide. Carbon monoxide is a dangerous air pollutant, but it is not counted among the major greenhouse gases, because it absorbs very little of the infrared heat leaving the earth. A greenhouse gas is one that traps this outgoing heat and warms the lower atmosphere. The major ones are carbon dioxide, methane, nitrous oxide, water vapour and the fluorinated gases such as HFCs, PFCs and sulphur hexafluoride. A is wrong because carbon dioxide, released by burning coal, oil and gas, adds the largest share to global warming. B is wrong because methane, which comes from paddy fields, cattle and landfills, traps far more heat per molecule than carbon dioxide does. D is wrong because nitrous oxide, mostly from nitrogen fertilisers, is a strong and long-lived greenhouse gas. Exam tip: carbon monoxide and sulphur dioxide are pollutants, not greenhouse gases; the gases listed in the Kyoto Protocol are carbon dioxide, methane, nitrous oxide and the F-gases.

The correct answer is D, Overgrazing. Overgrazing is a cause of soil degradation, not a way to conserve soil, so it is the option that does not belong. When too many animals feed on the same patch, the grass cover is eaten down to the roots and hooves loosen the bare topsoil, which wind and running water then carry away; in dry regions this is a main road to desertification. A is wrong as an answer because crop rotation is a genuine conservation method, since changing the crop each season restores nitrogen and other nutrients instead of draining one of them. B is wrong as an answer because terrace farming cuts a slope into steps, slowing runoff so that water soaks in rather than washing the soil down. C is wrong as an answer because tree roots bind the soil and shelter belts of trees break the force of the wind. Exam tip: contour ploughing, terracing, strip cropping and shelter belts conserve soil; overgrazing and deforestation destroy it.

The correct answer is D, Nitrogen. Nitrogen makes up about 78 per cent of the atmosphere, yet it is not a greenhouse gas. A molecule traps heat only if it can absorb outgoing infrared radiation, and that needs vibrations which change the spread of electric charge in the molecule. Nitrogen is a simple molecule of two identical atoms, so it is transparent to infrared; the same is true of oxygen. The gases that do trap heat are water vapour, carbon dioxide, methane, nitrous oxide, ozone and the halocarbons, and together they keep the earth about 33 degrees Celsius warmer than it would otherwise be. Option A is wrong because water vapour is the most abundant greenhouse gas of all. Option B is wrong because chlorofluorocarbons are powerful greenhouse gases that also destroy the ozone layer, which is why the Montreal Protocol phases them out. Option C is wrong because methane traps far more heat per molecule than carbon dioxide. Exam tip: nitrogen and oxygen are not greenhouse gases, though they make up most of the air.

The correct answer is D, New Delhi. The National Green Tribunal was set up by the National Green Tribunal Act, 2010, and started work on 18 October 2010 with its principal bench at New Delhi. Its four zonal benches sit at Bhopal, Pune, Kolkata and Chennai, and it also holds circuit sittings at other places. India thus became one of the first countries to create a dedicated court for environmental disputes, and the tribunal is required to decide cases within six months of filing. It hears matters under laws such as the Water Act, the Air Act and the Environment (Protection) Act, is headed by a chairperson who has been a judge of the Supreme Court or a High Court Chief Justice, and its orders can be appealed only in the Supreme Court. Option A is wrong because Kolkata holds the eastern zone bench, not the principal seat. Option B is wrong because Hyderabad has no bench of the tribunal. Option C is wrong because Lucknow is not the principal seat either. Exam tip: NGT Act 2010 — principal bench Delhi; zonal benches Bhopal, Pune, Kolkata and Chennai.

The correct answer is A, the zooxanthellae algae. A reef building coral polyp lives in a close partnership with single celled algae called zooxanthellae that live inside its own tissue. The algae photosynthesise and pass most of the food they make to the polyp, and they also give the coral its brown and green colour; the polyp in return gives them shelter and carbon dioxide. When the sea water becomes too warm or too acidic, or when it is muddied, the polyp expels these algae. The white calcium carbonate skeleton then shows through the transparent tissue, which is why the reef looks bleached, and unless the algae return the polyp starves. Option B is wrong because diatoms are free floating phytoplankton of open water and are not partners of coral. Option C is wrong because nitrogen fixing bacteria live in root nodules of legumes and in the soil. Option D is wrong because mycorrhizal fungi form partnerships with the roots of land plants, not with marine animals.