Fact 1: Mammals have a layer of fat under the skin to help insulate the body.

Fact 2: Glass insulation keeps beverage warm for hours.

Given the two facts above, answer the question "What keeps mammals warm?" with the following options: 
- evaporation
 - Animal fur
 - staying warm
 - sweating
 - They have thick fur
 - with their fur
 - a layer of fat
 - Veins and arteries.


 

       a layer of fat  