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
Ultrasound frequency used for diagnostic purposes in obstetrics - 
Select and explain. 
A. 1–20 MHz, B. 20–40 MHz, C. 40–60 MHz, D. 60–80 MHz
A: 1–20 MHz
Answer: A: 1–20 MHz 
Explanation: 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.