[单选题]
An observer approaches a9uxn+zj :nundx;cv78 stationary 1000-Hz sound source at twicets*77jum 0r3 y so a9eiq4r5;uhr-sr b the speed of sound. The obseassiuesj yro0;7u-btr r35mq*h4 r97rver hears a frequency of
A. 4000 Hz.
B. 2000 Hz.
C. 500 Hz.
D. none of the above
