Ultrasound frequency used for diagnostic purposes in obstetrics -: (A) 1–20 MHz, (B) 20–40 MHz, (C) 40–60 MHz, (D) 60–80 MHz
Answer is A. 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.