Fact 1: Thick fur can be used for keeping warm.

Fact 2: All mammals have fur or hair.

Given the two facts above, answer the question "What can be used for keeping warm?" with the following options: 
- electricity
 - Limbs
 - thick hair
 - sweat
 - ice
 - evaporation
 - cold blood
 - four limbs


 

   thick hair      