[单选题]
A sound source departs fros4l:xse oct6k6 p q(x5m a stationary observer. The frequency heard b ;2wf-vaxt shr 2c)wqjqg-x)0y the observerr- v2w )2acg0-qx )qfs tj;hwx is
A. higher than the source.
B. lower than the source.
C. the same as that of the source.
D. equal to zero.
