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

Fact 2: Carbon dioxide, CO2, is usually a harmless gas.

Given the two facts above, answer the question "what concentration in the air has increased dramatically over the last decade?" with the following options: 
- zinc
 - CO2
 - h2o
 - CFCs
 - lungs
 - coal
 - O2
 - CO 2


 

  CO2       