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': 'The number of muscle fibers innervated by a motor axon is smallest in: \nSelect and explain. \nA. Gastrocnemius, B. Orbicularis oculi, C. Single-unit smooth muscle, D. Soleus', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The extraocular and hand muscle has a ratio of 3-6 (total 3000 motor units) whereas gastrocnemius and back muscle (concern with posture) has a innervation ratio of 2000 (total 600 units).', 'role': 'assistant'}]