You are presented with the question "Which of the following is sometimes  used to indirectly heat buildings?" and the following answer choices: 
- hydrogen and oxygen
 - carbon monoxide
 - direct energy
 - dropping spheres
 - electromagnetic energy
 - a hydraulic system
 - nuclear fission
 - farm animals

Now knowing that electricity sometimes is used for heating buildings and nuclear fission is the only method currently used by nuclear plants to generate electricity, choose the best answer.


 

       nuclear fission  