The arithmetic of the Doppler Effect shows that if the distance between the source and observer is changing, then the observer hears a different frequency compared to the frequency actually radiating from the source.
Thus the first four choices would cause the student to hear a different frequency:
-- if the student walked toward the police car -- if the student walked away from the police car -- if the police car moved toward the student -- if the police car moved away from the student
The last two choices wouldn't affect the frequency heard by the student, since the perceived frequency of a sound doesn't depend on its intensity.
-- if the intensity of the siren increased -- if the intensity of the siren decreased.</span>
