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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?
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.
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.
The experiments showed that the caterpillars were not harmed by contact with the pollen from the genetically modified plants unless they ingested it.
The maize-eating insects that the natural insecticide protects against do not feed on the pollen of the maize plant.
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.
前提these caterpillars were fed milkweed leaves dusted with pollen from modified maize plants, they died.
结论Therefore, use of the modified maize inadvertently imperils monarch butterflies.
找削弱：如果毛毛虫吃不到覆盖毒花粉的叶子就不会减少了；或者毛毛虫吃的那个季节，没有花粉传播，或者没有风等等，都可以削弱。
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