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

Fact 2: Acidity increases as the pH number decreases, and alkalinity increases as the pH number increases.

Given the two facts above, answer the question "What can increase soil acidity?" with the following options: 
- Air pollution
 - Organic compounds
 - humans disturb it
 - cactuses
 - flooding
 - deforestation
 - Pollution
 - harmful substances


 

 Air pollution        