You are presented with the question "Concentrations of what in the air have increased over the last 10 years dramatically" and the following answer choices: 
- chlorofluorocarbons
 - fossil fuels
 - ozone
 - Greenhouse gases
 - Greenhouse gasses
 - oxygen
 - carbon dioxide
 - air molecules

Now knowing that carbon dioxide concentrations in the air have increased over the last decade dramatically and decades have 10 years, choose the best answer.


 

       carbon dioxide  