According to the doppler effect, which statement is true about a sound as it approaches you and then moves away.

a) as the source approaches you, the sound waves are compressed so the pitch of the noise increases.
b) the pitch of the sound remains the same the entire time, regardless of the direction of the source.
c) as the source moves away from you the sound waves are compressed so the pitch of the noise increases.
d) as the source approaches you, the sound waves are stretched relative to the individual so the pitch of the noise decreases.