7. An airplane traveling at half the speed of sound emits sound at a frequency of 5000 Hz. At what frequency does a stationary listener hear the sound as the plane approaches, and after it passes by? Assume the airplane is not flying very high.
(A) 5000 Hz, 6000 Hz
(B) 3000 Hz, 6666 Hz
(C) 4000 Hz, 2333 Hz
(D) 10000 Hz, 3333 Hz
(E) 7000 Hz, 9000 Hz

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