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': 'As deglutition involved voluntary and involuntary stages, so among the following, which structure limits the size of bolus swallowed: \nSelect and explain. \nA. Soft palate, B. Posterior pharyngeal pillars, C. Upper esophageal sphincter, D. Larynx', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]