题目选项分析

Though sucking zinc lozenges has been promoted as a treatment for the common cold, research has revealed no consistent effect. Recently, however, a zinc gel applied nasally has been shown to greatly reduce the duration of colds. Since the gel contains zinc in the same form and concentration as the lozenges, the greater effectiveness of the gel must be due to the fact that cold viruses tend to concentrate in the nose, not the mouth.

Which of the following, if true, most seriously weakens the argument?

【选项】The mechanism by which zinc affects the viruses that cause the common cold has not been conclusively established.