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?

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.
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.
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.
The sound of a boat engine generally travels much further under water

than it does through the air.
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.
答案选E 为什么呢？请指点！！谢谢

E是说在各种转速下，只要是背景音正常，这个动物总是听不清的。

所以这个方法并非是因为这个动物听不清所以失败，而是因为即使不降低速度，

背景音一来，这个动物总是分辨不了。所以如果有这样的情况，速度还是低点好.