UPSC CSE Prelims

Correct Option: D Explanation: Acid rain forms chiefly when sulphur dioxide (SO₂) and nitrogen oxides (NO and NO₂) form sulphuric and nitric acids in air; choice D is the intended exam response. Option-wise Analysis: A: Incorrect - Wrong: CO₂ makes weak carbonic acid in natural rain, while elemental N₂ is mostly unreactive. B: Incorrect - Wrong: CO and CO₂ are not the principal acid-rain precursors. C: Incorrect - Wrong: ozone and CO₂ mainly relate to other atmospheric chemistry, not dominant acid deposition. D: Correct - Intended correct option: sulphur dioxide and nitrogen oxides cause acid rain. Review Note: Option D says “nitrous oxide” (N₂O); the usual acid-rain precursor is nitrogen oxides NOₓ (especially NO/NO₂), not N₂O. This is a scientifically important terminology defect in the source question.