Why does the sound of something moving away from you seem to change to a lower and lower pitch?
a)
as the source moves away, the increased distance creates more interference; so the pitch drops.
b)
as the source moves away, the frequency of the sound waves is being compressed, so the pitch drops.
c)
as the source moves away, the sound waves speed up to reach the observer; this causes the pitch to drop.
d)
as the source moves away, the sound waves stretches out relative to the person standing still; so the pitch drops.