Answer: D (SO₂ and NO₂). Acid rain forms when acidic gases dissolve in rainwater. The two main culprits are sulfur dioxide and the oxides of nitrogen. Sulfur dioxide dissolves to form sulfurous acid, \(\text{SO}_2 + \text{H}_2\text{O} \to \text{H}_2\text{SO}_3\), and can be oxidised to sulfur trioxide which gives sulfuric acid. Nitrogen dioxide dissolves to give nitric and nitrous acids, \(2\text{NO}_2 + \text{H}_2\text{O} \to \text{HNO}_3 + \text{HNO}_2\).
Option A, carbon monoxide and carbon dioxide, is wrong: CO is essentially neutral and CO₂ gives only very weakly acidic carbonic acid, not classed as acid rain at O Level. Options B and C both include gases that are not acidic culprits — methane is not acidic and carbon monoxide is neutral. Only the pair sulfur dioxide and nitrogen dioxide is responsible for acid rain, so the answer is D.
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