Fact 1: Burning wood is used to produce heat.

Fact 2: Combustion-related, motor vehicles, wood burning, open burning, industrial processes.

Given the two facts above, answer the question "Combustion is used to produce what" with the following options: 
- energy
 - carbon
 - heat
 - earthquakes
 - air
 - oil
 - O2
 - water


 

   heat      