As an emergency vehicle approaches bob and moves away from jill, how does the actual frequency of the siren change? a) as an emergency vehicle approaches bob the frequency increases. b) as an emergency vehicle moves away from jill the frequency decreases. c) the actual frequency of the siren does not change despite appearances. d) bob does not notice any change until the vehicle passes him and the siren increases.

The correct answer is: 
C)The actual frequency of the siren does not change despite appearances.

In fact, Bob will observe an increase in the apparent frequency as the emergency vehicle approaches him, while Jill will observe a decrease in the apparent frequency as the emergency vehicle moves away from him, because of the Doppler effect (the relative velocity between the observer and the source of the sound is changing), but this effect involves the apparent frequency, while the real frequency of the siren will remain the same.