You are presented with the question "what concentration in the air has increased dramatically over the last decade?" and the following answer choices: 
- zinc
 - CO2
 - h2o
 - CFCs
 - lungs
 - coal
 - O2
 - CO 2

Now knowing that carbon dioxide concentrations in the air have increased over the last decade dramatically and carbon dioxide, CO2, is usually a harmless gas, choose the best answer.


 

  CO2       