Answer: b. the Doppler effect
Explanation:
Doppler effect describes the apparent change in the frequency heard by the listener than the frequency emitted by the source when there is relative motion between the two. When the source and listener approach each other, the apparent frequency is higher than the source and when they move away, the apparent frequency heard by the listener is lower than the source.
Due to Doppler's effect, a police siren sounds different when it comes closer than when it moves away. When it is closer, higher pitch can be heard than when it is moving past away.
