The argument is that zinc works better in the nose than in the mouth because the cold virus concentrates in the nose. The statement that weakens the argument will provide a different explanation for the efficacy of zinc in the nose compared to the mouth.

A. Experimental subjects who used the zinc gel not only had colds of shorter duration but also had less sever symptoms than did those who used a gel that did not contain zinc.out of scope - We are not interested in comparing zinc and no zinc, nor the specific effects of zinc.
B. The mechanism by which zinc effects the viruses that caused the common cold has not been conclusively established. The mechanism of zinc is out of scope.
C. to make them palatable, zinc lozenges generally contains other ingredients, such as citric acid, that can interfere with the chemical activity of zinc. This provides an alternative explanation for zinc's increased efficacy in the nose compared to the mouth. In the mouth zinc's efficacy is reduced by chemical activity with citric acid.
D. no Zinc based remedy can have any effect unless it is taken or applied within 48 hrs of the onset of cold symptoms. The time frame in which zinc-based remedies are applied is out of scope.
E. Drug company researchers experimenting with nasal spray based on zinc have found that it has much the same effect on colds as the gel does. There's no mention here of oral zinc remedies. This is only comparing two ways to apply zinc to the nose.