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

Fact 2: Radiators are used to heat the building, The energy source is oil.

Given the two facts above, answer the question "what is used to heat a building?" with the following options: 
- water
 - iron
 - coal
 - fire
 - gas
 - CFCs
 - oxygen
 - oil


 

        oil 