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
[{'content': 'Contractile unit of a muscle is? \nSelect and explain. \nA. Sarcomere, B. Sarcolemma, C. Myofibril, D. Sarcotubular System', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]