Fact 1: Harming an animal species causes that animal 's population to decrease.

Fact 2: Air pollution also harms plants and animals.

Given the two facts above, answer the question "What causes an  animal population to decrease?" with the following options: 
- sound pollution
 - Greenhouse gases
 - Pesticides
 - water
 - fossil fuels
 - Air pollution
 - animal diversity
 - steroids


 

      Air pollution   