Fact 1: Athermal insulator slows the transfer of heat.

Fact 2: Heat is a measurement of transferable energy.

Given the two facts above, answer the question "A thermal insulator slows the transfer of" with the following options: 
- moisture
 - energy.
 - voltage
 - sunlight
 - warmth
 - photons
 - matter
 - air


 

  energy.       