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?

1. 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.
2. 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.
3. 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.
4. The sound of a boat engine generally travels much further under water than it does through the air.
5. 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.

这题答案给的Ｅ．但我觉得Ｂ好．

Ｂ是削弱．结论在与低速没有解决问题，反而使问题更严重．要削弱这个结论．

Ｂ：高速的船会破坏鱼的事物Ｈ．　（也就是说低速是好的，不会破坏）削弱结论．

Ｅ错．　注意是使鱼接触at various speeds的时候，这些鱼失去了判断sound的能力．文中比较的是低速和高速．　这里的various不对．如果改为高速，还有点削弱的可能．

继续讨论．

B talks about the effect on manatees' food supply. The conclusion concerns the collision problem. B is out of the scope.

The conclusion is: boats slow down => sound lowers => manatees can't hear boats
E: manatees can't hear boats anyway, sound is not a factor that we should consider when evaluating the proposal