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
[{'content': "Carhart's notch seen at: \nSelect and explain. \nA. 0.5 KHz, B. 1 KHz, C. 2 KHz, D. 4 KHz", 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans: c (2 KHz) Ref: Dhingra, 3rd ed, p. 115 & p. 87Dip in bone conduction in otosclerosis & maximum occurs at 2000 Hz called Carhart notchDip Disappears after successful stapedectomy2 KHz dip - Otosclerosis4 KHz dip - Noise induced hearing loss8 KHz dip - Senile otosclerosis', 'role': 'assistant'}]