[单选题]
A sound source departs from a stationary observer. The frequency r9i +t 10 w ad*(3uxd6ypsycusco tj;(mrt,hwn z*oo., 2bk.hheard by the obser, .whnb.*r o,2h mztokver is
A. higher than the source.
B. lower than the source.
C. the same as that of the source.
D. equal to zero.
