The Doppler effect is an effect produced by a moving source of sound or electromagnetic waves due to the relative motion of a source or a receiver. If an emergency vehicle approaches Bob and moves away from Jill, how does the frequency of the siren change?

A) Neither Bob or Jill perceive any change in the frequency of the siren.
B) As an emergency vehicle approaches Bob the frequency appears to increase.
C) As an emergency vehicle moves away from Jill the frequency appears to increase.
D) Bob does not notice any change until the vehicle passes him and the siren increases.