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

Fact 2: Heat is produced by muscle action and shivering.

Given the two facts above, answer the question "What can mammals generate heat with?" with the following options: 
- Muscle action
 - with their fur
 - fur and fat
 - layers of fat
 - hair
 - Animal fur
 - waistlines
 - Bone action


 

 Muscle action        