Most acid precipitation results from the combination of with water in the atmosphere, forming strong acids that fall with rain or snow. see concept 3.3 (page 53)

The acid rain or the acid precipitation or the acid rain is the result of the combination of the water present in the atmosphere with the nitrogen oxides and sulfur oxides. The nitrogen oxides, and sulfur oxides when dissolved in water, forms strong acids, which falls along with the rain. The nitrogen oxides and sulfur oxides are present in the atmosphere as a result of the increasing air pollution.

Hence, the answer is 'nitrogen oxides and sulfur oxides'.