Reading Comprehension (RC) | Re: A stationary source of wave emissions propagates those waves into spac

It may be inferred that a moving observer could perceive the waves emitted by a moving object
A. at a velocity other than their real velocity
B. unless the object is traveling faster than the waves it emits
C. unless the medium through which the waves travel interferes
D. as if the object were stationary
E. unless the two are traveling away from each other at a speed different than the speed of the waves emitted by the object

I feel both A and D are correct . The question asks which COULD be true.
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