the Doppler effect. the Compton effect. a change in medium. a change in speed.
I am hesitating between 4 options to answer the following question, which option should I choose?
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
Possibilities:
- the Doppler effect.
- the Compton effect.
- a change in medium.
- a change in speed.

the Doppler effect.