You are presented with the question "What keeps mammals warm?" and the following answer choices: 
- fibers
 - eyeballs
 - white blood cells
 - fur
 - the skin
 - sweating
 - nostrils
 - heat

Now knowing that hair is a fiber that is found only in mammals and bats have hair like all mammals so they can stay warm, choose the best answer.


 

 fibers        