The underground water due to the hot rocks present inside the Earth ____________which can turn the turbines of generators to produce electricity.

The correct answer is A, Lichens. A lichen is a partnership of an alga and a fungus and takes its water and nutrients straight from the air, with no protective cuticle and no roots. It therefore absorbs dissolved sulphur dioxide along with moisture and dies even where the concentration is too low to harm most plants, so the disappearance of lichens from tree trunks in a city is read as a sign of sulphur dioxide pollution. Option B is wrong because earthworms indicate the health and moisture of soil. Option C is wrong because honey bees are watched as indicators of pesticide use and of the state of flowering plants. Option D is wrong because water hyacinth, the Bengal terror, indicates nutrient-rich polluted water rather than polluted air. Bio-indicator questions are best answered by asking which organism takes its living directly from the medium named, since that is the one which suffers first when the medium is fouled.

The correct answer is B, inactivates the catalyst in the converter. A catalytic converter holds platinum, palladium and rhodium on a honeycomb support, which oxidise carbon monoxide and unburnt hydrocarbons to carbon dioxide and water and reduce the oxides of nitrogen to nitrogen. Lead compounds from leaded petrol coat the surface of these metals and destroy their catalytic action, a process called catalyst poisoning, after which the converter no longer cleans the exhaust. Option A is wrong because the problem is chemical poisoning, not a rise in temperature. Option C is wrong because lead does not form a solid with carbon monoxide in the exhaust. Option D is wrong because tetraethyl lead was in fact added to petrol to raise its octane number, not to lower it. India therefore phased out leaded petrol before the tighter Bharat Stage norms, since the converter is the main device by which a modern engine meets them.

The correct answer is C, Particulate matter. An electrostatic precipitator gives the dust and ash particles in the flue gas an electric charge as they pass an electrode, and then collects them on plates carrying the opposite charge; the plates are shaken so that the collected ash falls into a hopper. It is the standard device for cleaning the chimney gases of a coal-fired power station and can remove the great majority of the particles. Options A and B are wrong because gases are not charged and collected this way; sulphur dioxide and the oxides of nitrogen are removed instead by a scrubber, in which the gas is washed with water or a lime slurry. Option D is wrong because carbon dioxide is not removed by either device in ordinary practice.

The correct answer is A, the Air (Prevention and Control of Pollution) Act, 1981. This Act is the specific law on air pollution. It empowers the Central and State Pollution Control Boards to lay down emission standards for industry and for vehicles, to declare air pollution control areas and to require consent before a polluting plant is set up or operated. Option B is wrong because the Environment (Protection) Act of 1986, passed after the Bhopal gas tragedy, is the umbrella law covering all media of the environment rather than air alone. Option C is wrong because the Water Act of 1974 concerns water pollution, though it is the Act under which the Central Pollution Control Board was first created. Option D is wrong because the Wildlife (Protection) Act of 1972 protects species and habitats.

The correct answer is B, Poor. The National Air Quality Index has six bands: 0 to 50 is Good, 51 to 100 Satisfactory, 101 to 200 Moderate, 201 to 300 Poor, 301 to 400 Very Poor and 401 to 500 Severe. A sub-index is computed for each pollutant for which enough data are available, and the worst of these sub-indices becomes the AQI of the day, the pollutant responsible being named as the prominent pollutant. Option A is wrong because Moderate covers 101 to 200. Option C is wrong because Very Poor begins only at 301. Option D is wrong because Severe is the topmost band, from 401 to 500, at which even healthy people are affected and emergency restrictions are usually imposed. The index is published for eight pollutants, so it is worth memorising the bands as three pairs: fifty and a hundred, two hundred and three hundred, four hundred and five hundred.