1. 1 The Doppler shift explains
(A) why the siren on a police car changes its pitch as it
races past us.
(B) why a sound grows quieter as we move away from
the source.
(C) how sonar works.
(D) the phenomenon of beats.
(E) why it is that our hearing is best near 3000 Hz.

2. 2 A sound source approaches a stationary observer.
The frequency heard by the observer is
(A) higher than the source.
(B) lower than the source.
(C) the same as that of the source.
(D) equal to zero.

3. 3
A sound source departs from a stationary observer. The frequency heard by the observer is
(A) higher than the source.
(B) lower than the source.
(C) the same as that of the source.
(D) equal to zero.