Fact 1: Mammals conserve body heat with their hair or fur.

Fact 2: Hair and feathers are both made of keratin.

Given the two facts above, answer the question "What do mammals use to conserve body heat?" with the following options: 
- four limbs
 - Energy.
 - energy
 - sarcasm
 - skin
 - keratin
 - sawdust
 - groceries


 

      keratin   