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 by a heating system?" with the following options: 
- engines
 - wind
 - insulation
 - photons
 - radiator
 - body heat
 - calcite
 - ice


 

     radiator    