the Doppler effect. the Compton effect. a change in medium. a change in speed.
I gave my students this multiple choice question: The driver of a car at a railroad crossing hears a change in pitch of the train whistle as the train passes by. This change in pitch is most likely due to

Only one answer is correct among these 4 choices:
- the Doppler effect.
- the Compton effect.
- a change in medium.
- a change in speed.

Could you tell me which one is correct?

the Doppler effect.