[单选题]
The Doppler shift expld(30dfn+9bwc bujk 3 nainsn-ue+ gvuifxl 82ul;8i ve it)j3 1*ku
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.