Fact 1: Mammals can also generate little bursts of heat by shivering.

Fact 2: Shivering causes muscle contractions to warm the body.

Given the two facts above, answer the question "Mammals can also generate little bursts of heat through what?" with the following options: 
- Movement of an air mass
 - crackling
 - Veins and arteries.
 - electromagnetic energy
 - house
 - fire
 - muscle contractions
 - aerobic capacity


 

       muscle contractions  