Fact 1: Coal is used for generating heat by burning.

Fact 2: CO 2 and SO 2 are both given off when coal is burned to provide heat or to generate electricity.

Given the two facts above, answer the question "When is coal burning _ is given off" with the following options: 
- CO 2
 - energy
 - vaporization
 - SO2
 - pollution
 - oxygen
 - carbon
 - smoke


 

 CO 2        