You are presented with the question "What contains energy to raise the temperture of an office?" and the following answer choices: 
- chemical messengers
 - peachleaf willow
 - deoxyribonucleic acid
 - cooling system
 - swimming pool
 - heating system
 - gym
 - organic molecules

Now knowing that energy is used for heating a building by a heating system and faculty offices are also in the building, choose the best answer.


 

      heating system   