[单选题]
The Doppler shift ex20q lb( f9o,2tw)ilygwf7dc wpi.rycyc4m6c . lains
A. why the siren on a police car changes its pitch as it races past us.
B. why a sound grows quieter as we move away from the source.
C. how sonar works.
D. the phenomenon of beats.
E. why it is that our hearing is best near 3000 Hz.
