Question

Fossil fuel combustion increases the acidity of rain because _____.

Answer 1

Fossil fuel combustion increases the acidity of rain because the sulfur dioxide is produced.

Because of fuel combustion,  sulfur dioxide goes up into the atmosphere as the hot gases rise, than it reacts with water and oxygen in the air and form sulfuric acid:
Balanced chemical reaction: SO₂(g) + 2O₂(g) + 2H₂O → 2H₂SO₄.