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

Fact 2: Research has shown that soil pH less than 6.0 can decrease crop yields.

Given the two facts above, answer the question "Air pollution can cause a decrease in what?" with the following options: 
- The atmosphere
 - kinetic energy
 - smoke
 - crop yields
 - nutrients
 - Temperature
 - fires
 - smog


 

    crop yields     