You are presented with the question "What can mammals generate heat with?" and the following answer choices: 
- Muscle action
 - with their fur
 - fur and fat
 - layers of fat
 - hair
 - Animal fur
 - waistlines
 - Bone action

Now knowing that mammals can also generate little bursts of heat by shivering and heat is produced by muscle action and shivering, choose the best answer.


 

 Muscle action        