Though sucking zinclozenges has been promoted as a treatment for the common cold, research hasrevealed no consistent effect.  Recently,however, a zinc gel applied nasally has been shown to greatly reduce theduration of colds.  Since the gelcontains zinc in the same form and concentration as the lozenges, the greatereffectiveness of the gel must be due to the fact that cold viruses tend toconcentrate in the nose, not the mouth.

In order to evaluate theargument, it would be most helpful to determine which of the following?

A.     Whether zinc is effective onlyagainst colds, or also has an effect on other virally caused diseases

B.     Whether there are remedies thatdo not contain zinc but that, when taken orally, can reduce the duration ofcolds

C.     Whether people who frequentlycatch colds have a zinc deficiency

D.     Whether either the zinc gel orthe lozenges contain ingredients that have an impact on the activity of thezinc

E.     Whether the zinc gel has aneffect on the severity of cold symptoms, as well as on their duration

如题，答案给的是C，可是我怎么也觉得C是无关选项，我选的是D

请大家帮忙分析一下，谢谢～

而且我发现GWD里面经常有些很不靠谱的答案，导致我有时对答案发现做错了，都不是很相信正确答案，不知道大家是不是也有一样的问题？