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: The US machine emits high frequency sound waves, ranging from 2-15 MHz, whose frequencies are considerably above the upper limit of human ear's audible range, i.e. greater than 20,000 Hz.", 'role': 'assistant'}]