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
[{'content': 'Cysteine is abundantly found in- \nSelect and explain. \nA. Keratin, B. Chondritin sulphate, C. Creatine, D. Spermine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The more the disulfide bond harder the keratin is.- Cysteine contributes to the disulfide bond.', 'role': 'assistant'}]