The main cause of acid rain is:

1. Methane and Oxygen
2. Sulfur Dioxide and Nitrogen Oxides
3. Carbon Monoxide and Helium
4. Hydrogen and Argon

Acid rain is caused by chemical reactions involving sulfur dioxide (SO2) and nitrogen oxides (NOx). These compounds are released into the air from burning fossil fuels in power plants and vehicles. They mix with water vapor to form sulfuric and nitric acid.