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: Radiology
[{'content': 'Ultrasound frequency used for diagnostic purposes in obstetrics : \nSelect and explain. \nA. 1-20 MHz, B. 20-40 MHz, C. 40-60 MHz, D. 60-80 MHz', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. 1-20 MHz', 'role': 'assistant'}]