[单选题]
An observer approaches a stationary 1000-Hz sound source at twice thjo gu144v4 s uq3y5 0ltxs51oj) fkxzxe speeqzf z5 93;bkpy3*i: hcdkq 2t+ex,nzb d of sound. ztz dpfz:k9+* qb3x ,k q5c3ih;2byneThe observer hears a frequency of
A. 4000 Hz.
B. 2000 Hz.
C. 500 Hz.
D. none of the above
