While standing on a sidewalk on a quiet day, you hear a tuning fork, held by a friend, produce a pure
440
hz
sound tone. when your friend sees you, she begins to ride her bike toward you. when she passes by you, she does not stop. instead, she continues biking away from you.
which statement is true while your friend is approaching you?

a. the sound intensity level of the sound wave emitted by the tuning fork increases.
b. the wavelength of the sound you hear increases.
c. you start to hear beats.

The sound intensity level of the sound wave emitted by the tuning fork increases.

Explanation:

When your friend is approaching you the sound of the tuning fork increases due to the Doppler effect.  The Doppler effect is the change in the frequency of the wave with respect to the observer who is moving towards the wave source.This is happened when the source of the wave moves towards the observer, each wave occur closer to the observer than the previous wave and the time taken is less to reach the observer than the previous wave.A wave should propagate when the velocity of observer and source are relative to medium which it is transmitted. Hence the frequency of the sound increases when approaches to the observer with a shorter wavelength.  

The answer is going to be c