The International Day for the Preservation of the Ozone Layer is observed on

The correct answer is A, all of the options. Each of the other three statements is correct on its own, which is exactly why the combined option is the answer. Option B is a true statement rather than the answer: ozone has three oxygen atoms, is unstable and is a strong oxidising agent, which is why it is used to disinfect water. Option C is also true: the ozone in the stratosphere absorbs most of the ultraviolet radiation from the sun and protects living things from skin cancer and cataract. Option D is true as well, but for ozone at ground level, where it is formed as a secondary pollutant when sunlight acts on nitrogen oxides and hydrocarbons from vehicles, and it is the main component of photochemical smog. Since B, C and D are each correct, no single one of them can be the answer and option A must be chosen. This good and bad character of ozone is the point being tested.

The correct answer is B, smog. A primary pollutant is released directly from a source, while a secondary pollutant is formed in the atmosphere when primary pollutants react. Photochemical smog is formed when sunlight acts on nitrogen oxides and volatile organic compounds to produce ozone and other irritants, so it is the classic secondary pollutant, and ground level ozone is counted with it. Option A is wrong because carbon monoxide comes straight out of an engine or a stove when fuel burns without enough oxygen. Option C is wrong because carbon dioxide is released directly by combustion and respiration. Option D is wrong because fly ash is the fine particulate matter that leaves the chimney of a thermal power station and is emitted as it is. The same distinction applies to acid rain, where sulphuric and nitric acids are secondary products formed from the primary gases sulphur dioxide and nitrogen oxides.

The correct answer is A, nitrogen oxide. Nitric oxide and nitrogen dioxide, together written as nitrogen oxides, are air pollutants that come mainly from combustion in vehicles and power stations; they help form smog and acid rain but are not counted among the greenhouse gases of the natural atmosphere. The gas of this family that is a greenhouse gas is nitrous oxide, a different compound, so read the name carefully. Option B is wrong as the answer because carbon dioxide occurs naturally from respiration, decay and volcanoes and is the principal greenhouse gas. Option C is wrong because methane is released naturally from wetlands and the digestion of animals and is a strong greenhouse gas. Option D is wrong because ozone occurs naturally in the stratosphere, is listed as a greenhouse gas, and also forms near the ground as a pollutant. The trap here is the similarity between nitrogen oxide and nitrous oxide.

The correct answer is B, stratosphere. About nine tenths of the atmosphere's ozone lies in the stratosphere, roughly fifteen to thirty five kilometres above the surface, where ultraviolet rays split oxygen molecules and the free atoms combine with other oxygen molecules to make ozone. The absorption of ultraviolet radiation there is also why temperature rises with height in this layer. Option A is wrong because the troposphere is the lowest layer, where weather occurs and where ozone appears only as a pollutant in smog. Option C is wrong because the mesosphere lies above the stratosphere, is the coldest layer and is where meteors burn up. Option D is wrong because the thermosphere is far higher, holds the ionosphere that reflects radio waves, and has air too thin for an ozone layer. Keep the order of the layers ready, since papers combine this with a question on where temperature rises or falls with height.

The correct answer is A, Dobson unit. The Dobson unit, named after the scientist who built the instrument for measuring atmospheric ozone, expresses how thick the ozone column would be if all of it were brought down to the surface at standard temperature and pressure; a hundred Dobson units correspond to a layer one millimetre thick. Option B is wrong because the curie is a unit of radioactivity, used for the rate at which a substance decays. Option C is wrong because the decibel measures the loudness of sound and is used in noise pollution standards. Option D is wrong because the angstrom is a unit of length equal to one ten billionth of a metre, used for atoms and for wavelengths of light rather than for a column of gas. Remember the Dobson unit together with the year 1985, when the thinning over Antarctica was reported.