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
C-shaped curve on tympanometry is seen in ? 
Select and explain. 
A. Serous otitis media, B. Otosclerosis, C. TM perfiration, D. Eustachian tube obstruction
D: Eustachian tube obstruction
Answer: D: Eustachian tube obstruction 
Explanation: Ans. is 'd' i.e., Eustachian tube obstruction