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': 'The ultrasonic sound waves employed in ultrasound machine for medical sonography have the frequency of - \nSelect and explain. \nA. 2 – 20 MHz, B. Less than 1 MHz, C. 20 – 20, 000 Hz, D. Greater than 100 MHz', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]