GWD-29-Q9，到底什么是断桥型削弱

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.

这里的E是正确答案，他说明无论开的快还是慢，其实都是没有用的，说明开的慢不会造成更加差的情况。所以对，

然后我看到别人的讨论里面说这种错法叫做断桥型削弱，请问到底什么是断桥型削弱，NN们能不能帮我解释下，最好再给我道例题，万分感谢了。

楼主复习到早上3点发的贴？先赞一下，鄙视一下自己不愿吃苦耐劳的精神。

结论：开的慢的船会让鱼撞死的更多

原因：鱼听不见低音调的慢船。

这两点要成立因果关系中间需要桥梁：即，鱼只要听见船的声音就会避开相撞事故

断开这座桥梁的削弱法：船开多快多慢鱼反正都是听不见的。

这道题我选的是Ｂ．但是看了Ｅ，觉得没问题．可是Ｂ为什么不对呢？Ｂ说了船开的慢的一个好处啊．削弱了结论说这个方法没用．

同问为什么B不正确？

原文论述的对象一致为boat的speed问题，E选项说的是sound，感觉完全是没有关系的呀？？

我选了B

其实B就是说船减速 有好处

E说船减速 没坏处

我觉得B的反驳更有力一些

本题situation：

问题在于：船撞鱼
        （文中很清楚的指出
        To address the problem)

解决措施：减慢船速

结论：因为：船减速è船的音调变低
        ＋
        鱼听不见低音

所以：减慢船速也不能减少撞鱼，反而会增加撞鱼（文中说 approach may
in fact make things worse rather than better.)

要削弱，就要紧扣结论。结论说会增加，那么削弱就要说不会增加。至于会不会减少，那不一定。即使不能减少，只要不增加，就算削弱结论。

        
1. 船需要减速的水域面积是如何决定的，跟船减速能不能减少撞鱼一点关系也没有。

        
2. 问题的中心是船撞鱼导致鱼死亡，而不是船使得鱼的食物减少。所以此为无关项。按照原来的结论，即使船减速了还是照样撞鱼，但是B却说船减速了可以让鱼有更多的食物。就算鱼有更多的食物吧，又怎么样呢？能削弱“船减速不能减少撞鱼”的结论么？所以B只是说了一个客观事实，也许这个事实是真的，但是不能削弱结论。

        
3. 船的数量增多，严重威胁了鱼类的生存。只是强调问题的严重性，不能说明提出的措施有没有效。

        
4. 船引擎的声音在说水中比在空气中传播更远。无关比较。

        
5. 实验中，鱼根本无法区分各种速度的船音和噪音。逻辑比较间接，要转一个弯。原文结论是从声音方面来说措施无效的：鱼听不见低速船的声音，就是说鱼的听觉变弱，所以船降低速度反而会增加撞鱼的概率。而E选项说：鱼不会分音调的高低。也就是说减速并不能使鱼的听觉变弱（起码和减速前差不多），撞鱼的概率起码不会像结论说的那样比原来增加。所以是削弱。