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: ENT
[{'content': 'Cause of U/L secretory otitis media in an adult is: \nSelect and explain. \nA. CSOM, B. Nasopharyngeal carcinoma, C. Mastoiditis, D. Foreign body of external ear', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]