This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Otosclerosis, Subject name: ENT
Carhart's notch in otosclerosis is observed at: (Repeat) 
Select and explain. 
A. 1000 HZ, B. 2000 HZ, C. 3000 HZ, D. 4000 HZ
B: 2000 HZ
Answer: B: 2000 HZ 
Explanation: Ans: B (2000HZ) Ref: Diseases of the Ear Nose and Throat by PL Dhingra 5th edn .2010.pg 97-100Explanation:Carhart's notchCarhart's notch is classically seen as a dip centered around 2 kHz range of hone conduction curve audiometery.This feature is seen in patients with otosclerosis.This notch will disappear after a succesful stapedotomyThere is also effective improvement in the patient's level of hearing at 2 KHz frequency levels.