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

Fact 2: Heating system is Gas.

Given the two facts above, answer the question "what energy is used for heating a building?" with the following options: 
- fire
 - flow of protons
 - wind
 - HPV
 - oil
 - h2o condensation
 - thermometers
 - gas


 

        gas 