The SI unit of radioactivity is:

The correct answer is B, Decibel. The decibel, written dB, is the unit in which the intensity level of sound is expressed, and it is named after Alexander Graham Bell. The scale is logarithmic, so every rise of 10 dB stands for a ten-fold rise in sound intensity; zero decibel is the threshold of hearing and about 120 to 130 decibel is the threshold of pain. For legal limits the A-weighted decibel is used because it follows the response of the human ear. A is wrong because the ohm is the unit of electrical resistance. C is wrong because the joule is the unit of work and energy, and although sound carries energy, its loudness is not reported in joule. D is wrong because the ampere is the unit of electric current. Candidates who confuse these pick an electrical unit instead of the acoustic one.

The correct answer is C, 2000. The Noise Pollution (Regulation and Control) Rules were notified in the year 2000 and they were framed under the Environment (Protection) Act, 1986. These Rules create the four zones, industrial, commercial, residential and silence zone, fix a day limit and a night limit for each, define day as 6 a.m. to 10 p.m. and night as 10 p.m. to 6 a.m., and restrict loudspeakers at night. A is wrong because 1986 is the year of the parent Environment (Protection) Act, which is the enabling law and not the noise rules themselves; this is the commonest trap in the question. B is wrong because 1991 is associated with the Public Liability Insurance Act. D is wrong because 2010 is the year of the National Green Tribunal Act, which created the tribunal that later heard noise cases.

The correct answer is B, 55 dB(A). The Rules of 2000 lay down 55 decibel as the day-time limit and 45 decibel as the night-time limit for a residential area. Reading the full table across is the safest way to answer such questions: industrial 75 by day and 70 by night, commercial 65 and 55, residential 55 and 45, and silence zone 50 and 40. A is wrong because 45 decibel is the night-time limit for the same residential zone, and the swapping of day and night values is the usual trap. C is wrong because 65 decibel is the day-time limit for a commercial area. D is wrong because 75 decibel is the day-time limit for an industrial area, the highest of the four zones. Day runs from 6 a.m. to 10 p.m.

The correct answer is B, 100 metres. The Rules define a silence zone as the area up to 100 metres around a hospital, an educational institution or a court, and such an area has to be declared by the competent authority. Inside it the limits are the strictest of the four zones, 50 decibel by day and 40 by night, and horns, drums, loudspeakers and firecrackers are not allowed. A is wrong because 50 is the decibel limit of the silence zone by day, not a distance, and the number is placed in the options to mislead. C is wrong because 200 metres has no place in these Rules. D is wrong because 500 metres is far wider than the Rules provide; the limit is deliberately kept small so that the zone can actually be enforced around a single building.

The correct answer is B, a fall in the dissolved oxygen content. The solubility of a gas in water decreases as the temperature of the water rises, so hot water discharged from a power station holds less dissolved oxygen. The damage is doubled because the metabolic rate of fish and other aquatic animals roughly doubles for every ten degrees of warming, so their demand for oxygen rises just as the supply falls. A is wrong because it states the relation in reverse; only cold water is rich in dissolved oxygen, which is why cold mountain streams support trout. C is wrong because salinity is raised by evaporation or by saline effluent, not by waste heat as such. D is wrong because a sharp fall in pH is caused by acidic effluent or acid rain, not by thermal discharge.