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: Physiology
How many polypepide chains does insulin compose of ? 
Select and explain. 
A. 4 chains, B. 3 chains, C. 2 chains, D. 5 chains
C: 2 chains
Answer: C: 2 chains 
Explanation: Insulin is two-chain polypeptide hormone, with 51 amino acids. Structure: 2 chains - A & B chain. A - chain - 21 amino acids; B - chain - 30 amino acids. Half-life-: About 5 mins