GWD29-9

Biris

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.
[*]不太明白为什么E对，另外为什么B不对呢？谢谢！

小气猫猫

这个就是说：海牛总是死于船只相撞。为了减少这个问题，就让船慢慢开。但是船慢了声音会比较低，所以海牛听不到比较低的声音，所以造成的后果更糟。

E weaken的是说，船无论快慢，海牛都听不到，所以造成的结果不会更糟（虽然也不会更好）。
B是一个无关选项。

不知道这么解释你理解不？

Caesar

读懂题就好理解了

NicoleYu

关键还是要找准原文的结论是什么：

this approach may in fact make things worse rather than better

也就是说原文的结论是减慢船速会让碰撞事故更多

E说其实不会有更多事故，因为速度快慢不影响海牛的听力，它们都听不到，这是一个有效削弱
B说到海牛的食物与船速的关系，但并没有联系到碰撞事故，因此是无关选项