This one is easy..
In an attempt to counter complaints that a certain pesticide is potentially hazardous to humans if absorbed into edible plants, the pesticide manufacturer has advertised that “ounce for ounce, the active ingredient in this pesticide is less toxic than the active ingredient in mouthwash.”
The argument says Ounce for Ounce contains less toxin than mouthwash, but we don't drink the mouthwash, we just wash the mouth. Immediately you can see that toxin, even if less concentrated, if ingested can make one sick. Lets see what options we have.
Which one of the following, if true, indicates a weakness in the manufacturer’s argument?
(A) The ounce-for-ounce toxicity of the active ingredient in mouthwash is less than that of most products meant for external use by humans, such as nail polish or other cosmetics.- this is beyond the realm of conclusion. and does not indicates a weakness. This is in fact in line with what the manufacturer is proposing.
(B) The quantity of toxins humans ingest by consuming plants treated with the pesticide is, on average, much higher than the quantity of toxins humans ingest by using mouthwash. - Bingo. Yes. If we are eating toxic plants even if those toxins are less concentrated than the toxin in mouthwash, it puts us at a risk. B looks good.
(C) The container in which the pesticide is packaged clearly identifies the toxic ingredients and carries warnings about their potential danger to humans. - Again just carrying warning does not preclude one from falling sick if he/she eats toxin loaded plants.
(D) On average, the toxins present in the pesticide take longer than the toxins present in mouthwash to reach harmful levels in the human body. - Okay. Does this indicate a weakness in the argument? No. Even if toxins takes longer than the toxins in mouthwash to reach harmful effect , it partially strengthens the argument. Maybe toxins in mouthwash starts harming us after 10 mins of gargling. But toxins from plant takes 10 days to show effect and indeed comes out from other end before it can do any nasty stuff in the body. :P :P funny. but a valid reasoning i guess.
(E) Since the government began to regulate the pesticide industry over ten years ago, there has been a growing awareness of the dangers of toxins used in pesticides.- This one misses the argument by a mile. Talks of random govt regulation instead of concentrating on conclusion.
SO the answer is B.