Fact 1: Carbon dioxide concentrations in the air have increased over the last decade dramatically.

Fact 2: CO 2 is carbon dioxide.

Given the two facts above, answer the question "What has increased in the air dramatically over the last decade?" with the following options: 
- oil
 - sugar
 - coal
 - Cars
 - H2O
 - CO 2
 - He
 - O2


 

      CO 2   