Fact 1: Burning fossil fuels increases the carbon in the atmosphere.

Fact 2: Increased atmospheric carbon dioxide increases greenhouse effect and warms the earth.

Given the two facts above, answer the question "What can warm the earth?" with the following options: 
- evaporation
 - burning fossil fuels
 - our nearest star
 - rain
 - electromagnetic energy
 - nuclear fusion
 - heat energy
 - magma


 

  burning fossil fuels       