UPSC NDA 2 2017 General Ability Solved Paper

Concept:Acid rain is caused by acidic oxides like sulphur dioxide and nitrogen oxides dissolving in rainwater, lowering its pH below 5.6.Explanation:Normal rainwater has a pH of about 5.6 due to dissolved $\mathrm{CO}_2$, which forms weak carbonic acid.When the pH falls below 5.6, it is called acid rain.The main pollutants responsible are sulphur dioxide ($\mathrm{SO}_2$) and nitrogen oxides ($\mathrm{NO}_x$).These oxides come from burning fossil fuels and industrial emissions.In the atmosphere, $\mathrm{SO}_2$ reacts with water and oxygen to form sulphuric acid ($\mathrm{H}_2\mathrm{SO}_4$), which falls as acid rain.Now, examine the given options:• Ozone ($\mathrm{O}_3$) is not a direct cause of acid rain.• Ammonia ($\mathrm{NH}_3$) is a base and can neutralise acids, not cause acid rain.• Carbon monoxide ($\mathrm{CO}$) is a toxic gas from incomplete combustion but does not form acid rain.• Sulphur dioxide ($\mathrm{SO}_2$) is an acidic oxide that directly contributes to acid rain formation.Therefore, only sulphur dioxide fits the cause.Answer:Sulphur dioxide (Option C)