Explanation

Reading the question: let's discuss a new way to attack a question, which is to focus on opinions. Words charged with opinion serve as guideposts to dissect an argument. Opinion is concentrated in the conclusion of an argument. When an argument is present, you can often use that fact alone to create a basic relevance filter.

Note: the connection between opinion and conclusion is no GMAT trick; it's more like a law of logic, almost a law of nature. Making an argument is the act of building bridges from things that are initially mutually agreed upon--"facts" or "evidence"--to things that are not initially agreed upon, but which you want ultimately to be agreed upon--and those things are "opinions" or "conclusions." When you catch a whiff of opinion, there likely is an argument present; and if you have an argument, there will absolutely be an opinion inside.

Creating a filter: "substantial" is an opinion word indicating the conclusion of the argument, and pieces of evidence are introduced by "because" and "moreover." Orienting by these words, we can see there are basically two kinds of evidence: a list of virtues of GMOs, and the idea that we would have spotted hazards in GMOs by now, if there were hazards. Both portions are facts, for the purpose of this argument; but the second one is weaker. Maybe we haven't looked in the right places? Or hazards are slow to set in? We'll use those ideas as predictions of the answer and look for something along those lines in the answer choices.

Applying our filter, we find (C) and (D) both roughly fit what we're looking for, and we notice that the other choices, while attacking GMOs, do not attack the argument--the connection of facts to opinion. We compare (C) and (D). One must be objectively better. Choice (D), even if true, is somewhat addressed already by the argument; we've put "lots of attention on them"--i.e., from outside of companies, so the secret would have gotten out. And if it's false, it doesn't help the argument much. (C), on the other hand, does help the argument if it's false, and it's closer to what we were thinking: maybe "all our attention" has been in the wrong place. The correct answer is (C).