You are presented with the question "What has increased in the air dramatically over the last decade?" and the following answer choices: 
- oil
 - sugar
 - coal
 - Cars
 - H2O
 - CO 2
 - He
 - O2

Now knowing that carbon dioxide concentrations in the air have increased over the last decade dramatically and cO 2 is carbon dioxide, choose the best answer.


 

      CO 2   