Which of the following is an example of the Doppler effect?
A. Two identical musical instruments being played next to each other will produce a sound twice as loud as if only one were played.
B. The siren of a police car sounds different when the car is coming toward you than it does when it passes you.
C. A hiker shouts "hello" into a canyon and hears it repeated back several times before it fades away.
D. Vibrations from a tuning fork pass through the air and cause a nearby dish of water to produce ripples of the same frequency.