You are presented with the question "What do mammals contain?" and the following answer choices: 
- food
 - ghosts
 - keratin
 - Animal fur
 - magic
 - soul
 - alveoli
 - four limbs

Now knowing that hair is a fiber that is found only in mammals and hair is largely made of keratin, a protein that contains a lot of the amino acid cystine , choose the best answer.


 

   keratin      