[单选题]
An observer approaches a stati.fgv /9bxii4ho9 ( cy0g mz0yconar c.)pvgn1ut:i/ gop0wy 1000-Hz sound source at twice the speed of sound. The )/.ow gtcv:0nipg1 up observer hears a frequency of
A. 4000 Hz.
B. 2000 Hz.
C. 500 Hz.
D. none of the above
