Which of the following could give rise to the Doppler Effect?

Question

Which of the following could give rise to the Doppler Effect?

A. Either the source of sound and/or the listener is/are moving.

B. Both the source of sound and the listener must be moving.

C. The source of sound is stationary and the listener must be moving.

D. The source of sound must be moving and the listener must be stationary.

Answer 1

Movement is always relative.

Therefore I have a problem with the question -- it uses the word "moving" without saying with respect to what.

Strictly speaking each of the sentences could be make true by judicious clause of the form "with respect to ..." added at the end of each sentence.

Doppler effect happens when source is moving relative to the listener.

If by "moving" the statement of the problem means "moving with respect to the ground" then A is the right choice.