Fact 1: Hair is a fiber that is found only in mammals.

Fact 2: Bats have hair like all mammals so they can stay warm.

Given the two facts above, answer the question "What keeps mammals warm?" with the following options: 
- fibers
 - eyeballs
 - white blood cells
 - fur
 - the skin
 - sweating
 - nostrils
 - heat


 

 fibers        