This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Ear, Subject name: ENT
Impedance audiometry is done using frequency probe of 
Select and explain. 
A. 220 Hz, B. 555 Hz, C. 440 Hz, D. 1000 Hz
A: 220 Hz
Answer: A: 220 Hz 
Explanation: It consists of a probe which fits into the external acoustic canal and has 3 channels: To deliver a tone of 220Hz To pick up the reflected sound a microphone To bring about changes in air pressure in the canal from positive to normal and then negative Ref: Dhingra; 6th Edition; pg no 24