Fact 1: Air pollution can cause the pH of soil to decrease.

Fact 2: Compost balances the pH of soil.

Given the two facts above, answer the question "What balances the effect of air pollution in soil?" with the following options: 
- wind
 - rubber
 - batteries
 - watershed
 - salinity
 - Earthworms
 - compost
 - acid


 

       compost  