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

Fact 2: Incinerated mercury pollutes the air.

Given the two facts above, answer the question "what can cause the ph of soil to decrease?" with the following options: 
- Greenhouse gasses
 - contamination
 - fossil fuels
 - chlorofluorocarbons
 - incinerated mercury
 - harmful substances
 - Greenhouse gases
 - Temperature


 

     incinerated mercury    