This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Cysteine is abundantly found in- 
Select and explain. 
A. Keratin, B. Chondritin sulphate, C. Creatine, D. Spermine
A: Keratin
Answer: A: Keratin 
Explanation: The more the disulfide bond harder the keratin is.- Cysteine contributes to the disulfide bond.