What should be the temperature difference between the upper layers and deeper layers of the ocean for setting up the OTEC power plant?

The correct answer is B, hydrofluorocarbons. Hydrofluorocarbons were adopted as substitutes for chlorofluorocarbons in refrigeration and air conditioning because they do not harm the ozone layer, but they turned out to be very powerful greenhouse gases, so the Kigali Amendment of 2016 brought them under the Montreal Protocol with a schedule for reducing their use. Option A is wrong because chlorofluorocarbons were covered by the original protocol of 1987 and their production has already been phased out under it. Option C is wrong because sulphur dioxide, the main cause of acid rain, is controlled by national air quality laws and emission standards, not by this treaty. Option D is wrong because methyl bromide, used to fumigate soil, is also part of the original list of ozone depleting substances. The point being tested is that Kigali deals with a climate gas through an ozone treaty.

The correct answer is C, 16 September. The day marks the signing of the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987 and is observed every year with a theme announced by the United Nations. Option A is wrong because the fifth of June is World Environment Day, which marks the opening of the Stockholm Conference of 1972. Option B is wrong because the twenty second of April is Earth Day. Option D is wrong because the second of December is observed in India as National Pollution Control Day, in memory of the Bhopal gas tragedy of 1984. Environment sections regularly place these four dates in one question, so learn them as a set along with the twenty second of May for biological diversity and the second of February for wetlands.

The correct answer is A. Marble is calcium carbonate, and sulphur dioxide in polluted air, along with the sulphuric acid of acid rain, reacts with it to form calcium sulphate, so the stone loses its shine, turns yellow and becomes pitted and flaky. This damage is called marble cancer, and it is discussed most often in connection with the Taj Mahal at Agra, which is why industries around it are regulated. Option B is wrong because the lung disease caused by inhaling stone dust is silicosis, a different subject altogether. Option C is wrong because algal growth is a biological stain that can be cleaned and is not what the term means. Option D is wrong because thermal cracking is a physical weathering process common to many rocks and has nothing to do with pollution. Remember that any monument of marble or limestone is vulnerable, since both are calcium carbonate.

The correct answer is B, sulphur dioxide and oxides of nitrogen. These gases come from burning coal and oil in thermal power stations, smelters and vehicles, are oxidised in the atmosphere and dissolve in water droplets to form sulphuric acid and nitric acid, which come down as acid rain. Option A is wrong because carbon dioxide and methane are greenhouse gases; carbon dioxide does dissolve to form weak carbonic acid, but that only makes normal rain slightly acidic at a pH near 5.6 and is not what is meant by acid rain. Option C is wrong because carbon monoxide is a poisonous gas that does not form a strong acid, and ozone at ground level is an irritant in smog rather than a source of acidity. Option D is wrong because chlorofluorocarbons and halons destroy stratospheric ozone and are the concern of the Montreal Protocol, a separate problem from acid rain.

The correct answer is B, substances that deplete the ozone layer. The Montreal Protocol was adopted in 1987 under the Vienna Convention of 1985 and sets the timetable by which countries must stop producing and using chlorofluorocarbons, halons, carbon tetrachloride, methyl chloroform and methyl bromide. Option A is wrong because greenhouse gas emissions are the subject of the United Nations Framework Convention on Climate Change and the agreements under it, although the Kigali Amendment of 2016 did bring hydrofluorocarbons, which are greenhouse gases, into the Montreal Protocol. Option C is wrong because the movement of hazardous waste across borders is governed by the Basel Convention. Option D is wrong because wetlands are covered by the Ramsar Convention. Matching each convention to its single subject is the fastest preparation, since one such question appears in nearly every environment section.