Fact 1: Electricity sometimes is used for heating buildings.

Fact 2: Nuclear fission is the only method currently used by nuclear plants to generate electricity.

Given the two facts above, answer the question "Which of the following is sometimes  used to indirectly heat buildings?" with the following options: 
- hydrogen and oxygen
 - carbon monoxide
 - direct energy
 - dropping spheres
 - electromagnetic energy
 - a hydraulic system
 - nuclear fission
 - farm animals


 

       nuclear fission  