Answer:
Apparent frequency increases for Bob and decreases for Jill.
Explanation:
Doppler effect refers to the apparent change in frequency when there is relative motion between source and the listener. When the source and listener move closer, the apparent frequency heard by the listener increases. And when the source and listener move away from each other, the apparent frequency heard by the listener decreases.
Thus, when the emergency vehicle approaches Bob, the apparent frequency of the siren increases. It moves away from Jill, the apparent frequency heard by the Jill decreases.
