GWD-12-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.     
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.

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.

问问C为什么错？