You are presented with the question "What is used to heat a building by a heating system?" and the following answer choices: 
- engines
 - wind
 - insulation
 - photons
 - radiator
 - body heat
 - calcite
 - ice

Now knowing that energy is used for heating a building by a heating system and radiators are used to heat the building, The energy source is oil, choose the best answer.


 

     radiator    