They no probably no longer eat the same things, or one of the populations has adapted to its environment and the other flies continue to be normal.
Answer:
Gene expression is primarily controlled at the level of transcription, largely as a result of binding of proteins to specific sites on DNA. ... The regulator gene codes for synthesis of a repressor molecule that binds to the operator and blocks RNA polymerase from transcribing the structural genes.
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IAMTHEGOAT
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The answer you were looking for is....
(D) Nonpoint source pollution is difficult to regulate, because its source is difficult to track.