Consider the following: 1. Carbon monoxide 2. Nitrogen oxide 3. Ozone 4. Sulphur dioxide. Excess of which of the above in the environment is/are cause(s) of acid rain?

1. Carbon monoxide — CO is a toxic pollutant, but it is not a major acid-rain forming gas. Verdict: Incorrect.

2. Nitrogen oxide — Nitrogen oxides react with water and oxygen in the atmosphere to form nitric acid, a major component of acid rain. Verdict: Correct.

3. Ozone — Ozone is a pollutant at ground level, but it does not directly cause acid rain. Verdict: Incorrect.

4. Sulphur dioxide — SO2 oxidizes in the atmosphere to form sulphuric acid, the principal cause of acid rain. Verdict: Correct.

So, 2 and 4 only are causes of acid rain.