Insecticide Q is noteworthy both for its control of pests that attack crops and its benignity toward humans.


Ok: here we have a double benefit of the chemical

However, its effects only last 30 days, a period insufficient to protect any major crop until harvest, and reapplication of Insecticide Q usually kills the crop.

here we do have a downside: only one month the protection lasts and a reapplication would kill the crop. USUALLY

Insecticide P, on the other hand, can be reapplied indefinitely without damaging the crop. Therefore, Insecticide Q has no discernible use in the protection of major crops.

Ok. The P compound instead can be reapplied indefinitely but maybe in the first 30 days does not have the same positive effects. We do not know. Therefore, Q is not usable according with the argument

We need to find an option that tells us P has some flaw or that Q is useful indeed. Notice how therefore indicates us the conclusion of the argument and this is what we must attack


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

A) There are several minor crops that can be harvested in as few as 20 days.

This option does not tell us anything of useful. The fact that minor crops can be harvested before the month not indicate something about if Q is good or not. we already know that Q lasts for 30 days. Moreover, it is not specified IF this is the first or the second application or else

B) A third insecticide not mentioned in the passage protects crops better than either Insecticide Q or P.

Completely OFF as an option. A third compound is NOT what we are looking for to undermine the argument itself

C) Foods treated with Insecticide P are often banned from organic markets.

Stupid option. We need to find something related to the treatment of the crops NOT the banning of food

D) Both Insecticides Q and P can be used on the same crop.

This tells us that Q is useful and effective. For example: we can use it in the first month to protect the crop and THEN use P for a reapplication until we harvest the crop BOTh on the same plant and also both are not harmful for humans. Sooo Q is any good, contrary to our conclusion

E) The production of Insecticide P, once economical, has recently tripled in cost.

Irrelevant

I hope this helps