Critical Reasoning

Plantings of cotton bioengineered to produce its own insecticide against bollworms, a major cause of crop failure, sustained little bollworm damage until this year. This year the plantings are being seriously damaged by bollworms. Bollworms, however, are not necessarily developing resistance to the cotton's insecticide. Bollworms breed on corn, and last year more corn than usual was planted throughout cotton-growing regions. So it is likely that the cotton is simply being overwhelmed by corn-bred bollworms.

In evaluating the argument, which of the following would it be most useful to establish?
A. Whether corn could be bioengineered to produce the insecticide
B. Whether plantings of cotton that does not produce the insecticide are suffering unusually extensive damage from bollworms this year
C. Whether other crops that have been bioengineered to produce their own insecticide successfully resist the pests against which the insecticide was to protect them
D. Whether plantings of bioengineered cotton are frequently damaged by insect pests other than bollworms
E. Whether there are insecticides that can be used against bollworms that have developed resistance to the insecticide produced by the bioengineered cotton

The OA to this question is B.However, I am unable to get out of the following confusion:
In such question types:which of the following would it be most useful to establish?
If I am right, we look for choices that either strengthens or weakens the argument.(?)
From that point of view,B and D are both relevent.
If the Answer to D is yes, then the argument weakens.if the answer is No,the argument is strengthened.
How do you make D irrelevent ?

Thanks in Advance.