[单选题]
A sound source (normal frequency of *mvlg7-3 zjn 4hn9nb p1000 Hz) approaches a stationaqj49 ew) .p.qpelvx1.3kzyhb ry observer at one-half the speed of sound. The observer hears a frequenh4)vwj epq.1.3lpyeb z 9kq.x cy of
A. 2000 Hz.
B. 500 Hz.
C. 1500 Hz.
D. none of the above
