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

Fact 2: Hair is largely made of keratin, a protein that contains a lot of the amino acid cystine .

Given the two facts above, answer the question "What do mammals contain?" with the following options: 
- food
 - ghosts
 - keratin
 - Animal fur
 - magic
 - soul
 - alveoli
 - four limbs


 

   keratin      