请教GWD12中的几道逻辑题，谢谢

ukysky

Q16:
Certain genetically modified strains of maize produce a natural insecticide that protects against maize-eating insects.  The insecticide occurs throughout the plant, including its pollen.  Maize pollen is dispersed by the wind and often blows onto milkweed plants that grow near maize fields.  Caterpillars of monarch butterflies feed exclusively on milkweed leaves.  When, in experiments, these caterpillars were fed milkweed leaves dusted with pollen from modified maize plants, they died.  Therefore, use of the modified maize inadvertently imperils monarch butterflies.

Which of the following, if true, most seriously weakens the argument?

A. er unit of volume, the natural insecticide produced by the genetically modified maize plants is less toxic to insects than are many commercial insecticides commonly used on maize plants.
B. Standard weed-control practices that have been used by farmers for decades have largely eliminated milkweed plants from certain areas where monarch-butterfly caterpillars were once common.
C. The experiments showed that the caterpillars were not harmed by contact with the pollen from the genetically modified plants unless they ingested it.
D. The maize-eating insects that the natural insecticide protects against do not feed on the pollen of the maize plant.
E. Airborne maize pollen tends to collect on the middle leaves of milkweed plants and monarch caterpillars feed only on the plant’s tender upper leaves.

答案是E，可是既然花粉是被风吹到milkweed leave上的，monarch caterpillar吃上层的树叶不是更容易被感染到吗，不知道这里是怎么形成对原文结论的weaken的

Q29:

The Earth’s rivers constantly carry dissolved salts into its oceans.  Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the oceans to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth’s oceans can be accurately estimated.

Which of the following is an assumption on which the argument depends?

A. The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
B. At any given time, all the Earth’s rivers have about the same salt levels.
C. There are salts that leach into the Earth’s oceans directly from the ocean floor.
D. There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans.
E. None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.

答案是E，我选了A

即使将E所说成立，即海洋里的盐分确实被生物消耗，这种消耗也是持续的，已经被计算到past hundred years的盐分变化中了，因此如果根据past hundred years的盐分变化速度推测海洋年龄，这种假设应该对结论不构成额外影响，作这样的假设似乎并没有多大意义

而A对于过去几百年海洋盐分的增长，用了not been unusually large这个限定词，正对应了原文结论中的maximun age，如果过去几百年的增长速度没有特别快，那么即使特别慢，得到的也只是一个偏大的ocean age，与原文的maximun age看似并不矛盾

不知道我表达清楚了没有，还有一周要考试了，请各位提供我一些指点和提示，谢谢了！

atc

同学，逻辑题目要把原文看仔细呀：

Q16：E. Airborne maize pollen tends to collect on the middle leaves of milkweed plants and monarch caterpillars feed only on the plant’s tender upper leaves

想想一下现实生活中的吃河豚，明白了吧。

Q29：题目给了一个计算海洋年龄的模型，注意到The Earth’s rivers constantly carry dissolved salts into its oceans。

[此贴子已经被作者于2008-2-28 20:22:36编辑过]

pisceswzh

Q16: 答案中说花粉掉在了中间的叶子上，而毛毛虫吃的是上面的叶子，所以应该是不会感染的，从而对原文说将会受到感染做了weaken.

pisceswzh

Q29

[此贴子已经被作者于2008-2-29 16:07:11编辑过]