You are presented with the question "What can be used for heating a building by a heating system?" and the following answer choices: 
- the Sun's rays
 - peachleaf willow
 - engines
 - oxygen
 - photosynthesis
 - photons
 - air
 - hydrogen and oxygen

Now knowing that energy is used for heating a building by a heating system and solar energy is the energy in the sun's rays, choose the best answer.


 

 the Sun's rays        