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: Unknown
[{'content': "The commonly used frequency range in obstetrical ultrasonography is - \nSelect and explain. \nA. 3.5-5 KHz ', B. 20-25 KHz, C. 3.5-5 MHz, D. 20-25 MHz", 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans: C', 'role': 'assistant'}]