I got this one wrong so here's my breakdown:
Sentence 1: Contextual Question? Should gov't abandon toxicity testing?
Sentence 2: Conditional language introduced: Abandon > Accpt Level = 0
Sentence 3: Virt. all have some toxicity ( i.e. almost none have 0)
Sentence 4 & 5: Moreover, we can never be certain of 0 all we can know is below threshold of current analytics
Core: Since we can't figure out toxicity threshold > gov't should still attempt to determine at what levels to allow toxic substances in our food supply.
A) Bingo.
B) No, it's not that it needs to be zero but that we should continue to determine whether the level of toxicity which is consumed can be acceptable at certain threshold levels that are NOT zero, since virtually all foods contain some level of toxicity. This has it backwards.
C) If this were the main point, why begin with a question about the gov't? If we were to say that this is the main point then the first two sentences would be rendered irrelevant. While this can be the case in some arguments, this argument's introductory question is germane to the rest of the argument and thus this cannot be the correct answer.
D) This is the answer choice I fell for. Unfortunately upon closer inspection the stimulus clearly says that CURRENT analytical methods cannot determine the threshold. Does that mean they will NEVER be able to determine the threshold? We have no idea. Thus this cannot be the correct answer.
E) Again this one is tempting. At first glance, refining the methods seem plausible considering the fact we are having difficulty determining the limits of toxicity in foods -- however what are the current methods? Do we even know? If we don't even know what the methods are then how can we be saying that the main point is to change the methods? Obviously this is not the main thrust of the argument and thus can be confidently eliminated.
Hope this helps!