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

Fact 2: Solar energy is the energy in the sun's rays.

Given the two facts above, answer the question "What can be used for heating a building by a heating system?" with the following options: 
- the Sun's rays
 - peachleaf willow
 - engines
 - oxygen
 - photosynthesis
 - photons
 - air
 - hydrogen and oxygen


 

 the Sun's rays        