[单选题]
A sound source (normal frequency of 1000 Hz) approache- l1 6r9svnhsns a stationary observeqp 8*c;bvdr;e+ /oymar at one-half the speed of sound. The obs;aqdo+vre 8 p/y ;bc*merver hears a frequency of
A. 2000 Hz.
B. 500 Hz.
C. 1500 Hz.
D. none of the above
