Answer #19

The correct answer is D. You need to know the Doppler shift equation to answer this question as shown.
You also need to have a rough sense of the difference in ultrasound propagation velocity for different media with blood being the fastest and lung tissue (mostly air) being the slowest and blood being in the middle.

Changing the angle by only 20 degrees will only change the shift by about 6 percent (Cosine of 0 is 1 and cosine of 20 is 0.94) Changing the velocity is a 2 fold difference and changing the transducer frequency also exactly 2 fold. However since the speed of ultrasound in lungs is only 500 m/s and in blood it is roughly 1500 m/s this is a 3 fold change. It will impact the Doppler shift the most and hence is the correct answer.