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?
题目的问题核心是Therefore, use of the modified maize inadvertently imperils monarch butterflies，就是说危害了，而没有说危害了多少。花粉落在milkweeds的中层，而幼虫只吃最上层的叶子，所以它死不死和 Maize pollen一点关系也没有。
When, in experiments, these caterpillars were fed milkweed leaves dusted with pollen from modified maize plants, they died. 如果吃了-死。那隐藏key就算最终吃了没有。所以削弱有两个方面。1是削弱中间的过程：如由于某种原因没吃上（比如选项E/有些选项说是 由于花粉飘散存在季节性，毛毛虫吃叶子的时候刚好不是哪个季节）2是直接削弱结论：比如由于存活的第二代产生抗体，不再对其敏感-直接削弱结论Imperil
A.Per 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. 本选项说同样的量玉米的杀虫剂比商用的杀虫剂毒性弱，属于上面所述的转移焦点(转移焦点至题干可能提及，或未曾提及之事物)，与题目论点——玉米会杀M幼虫无关 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. 本选项说由于控制杂草milkweed在这个地区减少了很多，所以虫子也不剩多少了，同样属于转移焦点，与题目论点无关。 C.The experiments showed that the caterpillars were not harmed by contact with the pollen from the genetically modified plants unless they ingested it. 本选项说M幼虫只要不吃花粉就不会死，除此之外爱怎么接触怎么接触，总之就是死不了;说到吃花粉会死，那也就还是玉米会危及M幼虫，支持了结论。但是题目求的是weaken，所以错误啦。 D.The maize-eating insects that the natural insecticide protects against do not feed on the pollen of the maize plant. 本选项说吃玉米的虫子不会吃花粉，还是转移焦点，无关题目结论。 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. 正解。本选项说花粉会落在中间的叶子上，而M幼虫只会吃上面的叶子，总之一句话，随你怎么飘，就是不吃你。这样一来，虽然玉米花粉飘到milkweed上了，但是M幼虫既没有接触到花粉也没有吃，所以完全不会威胁到生存，也就是否定了题目的果——因果关系中的结果不会产生，故而削弱。