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
Contractile unit of a muscle is? 
Select and explain. 
A. Sarcomere, B. Sarcolemma, C. Myofibril, D. Sarcotubular System
A: Sarcomere
Answer: A: Sarcomere 
Explanation: None