Though sucking zinc lozenges has been promoted as treatment for the

Hi all, here's an answer to this tricky question from one of our CR experts at Knewton, Jonathan Bethune. I hope it's helpful!

This is a very interesting question. The difficulty stems from the fact that the correct answer does not appear to have any direct bearing on the argument.

Start by eliminating the obviously wrong answer choices. Choice A adds details that do not undermine anything in the argument. Choice B is too weak – we don’t really need to know the mechanism by which zinc effects viruses in order to know that it helps beat the cold. Choice D is a random detail. Choice E strengthens the argument because it shows that other zinc-based nasal treatments work just as well as the gel.

Even if you rule out all of the other answer choices, how can we deduce that C is correct? The key here is to keep in mind what the specific argument is: Zinc gel works better than zinc lozenges because cold virus concentrate in the nose. A condition of this argument is the fact that lozenges and gel contain the same concentration of zinc. If the gel had more zinc than the lozenge, then the higher concentration of zinc might account for its increased efficacy.

This is basically what choice C does. It undermines the assumption that both medications contain the same concentration of zinc. If the citric acid in the lozenge is interfering with the zinc, then it is possible that the gel works better simply because its zinc is NOT being interfered with. A lozenge with no citric acid (which presumably would taste awful) might work just as well as the gel. Therefore we cannot assume that the gel works better because cold virus is in the nose. It may be the lack of citric acid that makes the gel work better.

Therefore C is correct.