How will the right answer fit in terms of support and conclusion?

Only the right answer will support disagreeing with the conclusion.

Highlight the main conclusion in the passage, if there is one:

this development raises the possibility that the drugs will end up in the general food supply

Consumer advocate: [BACKGROUND]. However, [CONCLUSION], since [SUPPORT].

This author tried to be sneaky and change the subject, but you caught it. The conclusion brings in “the general food supply”, which isn’t mentioned anywhere else. The right answer will call out that this whole thing about how a “crop drifts into a nearby field” doesn’t have anything to do with the “food supply”.

Map the wording of the answers to the wording of the passage:

(A) …if they were present in the general food supply.

Be careful. You don’t care what happens after the drugs get into the food, only if it’s possible. The author never actually says anything about “danger to public health”.

(B) …is prevented from drifting into fields…

Cool story, but that’s not the situation in the passage. The author didn’t say anything about the drifting being “prevented”.

(C) …the crop species that comprise the largest portions of the general food supply.

That’s getting warm, but that doesn’t say the drugs can’t get into the food supply at all. The conclusion is really weak. It just says there’s “the possibility” of drugs in the food supply. To convince us to disagree we need an answer that shows it’s impossible, so this answer isn’t strong enough.

(D) …the drugs are not present in any part of the plant used for food…

This is pretty clearly saying the drugs aren’t in the food supply. That would directly undermine the argument the author is making. And it doesn’t add in any details that don’t re-use wording used in the passage. This is gold.

(E) …it would be possible for scientists to identify the latter crop.

If you like this one, you’re probably thinking that scientists could then prevent the drugs from getting into the food supply. But that isn’t plain common sense. You’re assuming that would be the case. If you catch yourself doing that, you’ve proven this is a wrong answer.

(D) is the correct answer.