Energy is used for heating a building by a heating system, and solar energy is the energy in the sun's rays. Given these facts, what can be used for heating a building by a heating system among the following options:
- the Sun's rays
 - peachleaf willow
 - engines
 - oxygen
 - photosynthesis
 - photons
 - air
 - hydrogen and oxygen


 

 the Sun's rays        