GWD-29-Q9 的疑惑，求NN解答

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Manatees, aquatic mammals inhabiting Florida
’s rivers and coastal waters, swim close to the surface and are frequently killed in collisions with boats.To address the problem, boat traffic in manatee-populated waters is being required to maintain very low speeds.Unfortunately, manatees are unable to hear low-pitched sounds and a boat’s sound lowers in pitch as the boat slows.Therefore, this approach may in fact make things worse rather than better.
Which of the following, if true, casts most doubt on the conclusion?

A.The areas where boats would have to maintain low speeds were decided partly on the basis of manatee-population estimates and partly from numbers of reported collisions between manatees and boats.

B.Because the water hyacinth that manatees feed on grows best in water that is nearly still, water hyacinth beds can be disturbed or damaged by fast-moving boat traffic.

C.Over the last several decades, boat traffic in Florida
’s coastal waters has been increasing almost continuously and now represents the greatest threat to the endangered manatee population.
D.The sound of a boat engine generally travels much further under water than it does through the air.

E.When experimenters exposed manatees to the recorded sounds of boats moving at various speeds, the creatures were unable to discern the sounds over normal background noise.


E中的discern the sounds over normal background noise.
是不能区分声音和背景噪音还是不能那些超过背景噪音的声音？
谢谢

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