Options are:
[1]. fibers
[2]. eyeballs
[3]. white blood cells
[4]. fur
[5]. the skin
[6]. sweating
[7]. nostrils
[8]. heat
Given those answer options, answer the question: 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, what keeps mammals warm?
A:
[1].