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': 'C-shaped curve on tympanometry is seen in ? \nSelect and explain. \nA. Serous otitis media, B. Otosclerosis, C. TM perfiration, D. Eustachian tube obstruction', 'role': 'user'}, {'content': "Answer: D \nExplanation: Ans. is 'd' i.e., Eustachian tube obstruction", 'role': 'assistant'}]