Fact 1: Mammals generate heat mainly by keeping their metabolic rate high.

Fact 2: Aerobic exercise also contributes to the increased the metabolic rate.

Given the two facts above, answer the question "Mammals can generate heat by" with the following options: 
- hibernation
 - layers of fat
 - perspiration
 - aerobic exercise
 - respiration
 - exercise
 - with their fur
 - to stay warm


 

    aerobic exercise     