Answer #3

The correct answer is C: 3 cm.

Recall that the pulse repetition period (PRP) is the time that passes between pulses sent out by the transducer. In between pulses the transducer is listening back for signal. The longer the depth, the more time will be needed to listen and hence the PRP will be longer.

As a general useful rule of thumb, in human tissue (assuming a propagation velocity of 1540 meters/second), the depth is 1 cm for every 13 microseconds of a pulse repetition period. In this case since we are told the PRP is 39 microseconds, the depth is 39/13 or 3 cm.

In fact, any time you are given the depth, you also should be able to calculate the PRP (just multiply the depth in cm by 13 and you will have your answer in microseconds). This is one variation of how this type of question could be asked.