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