Fact 1: Energy is used for heating a building by a heating system.

Fact 2: Faculty offices are also in the building.

Given the two facts above, answer the question "What contains energy to raise the temperture of an office?" with the following options: 
- chemical messengers
 - peachleaf willow
 - deoxyribonucleic acid
 - cooling system
 - swimming pool
 - heating system
 - gym
 - organic molecules


 

      heating system   