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? ['Experimental subjects who used the zinc gel not only had colds of shorter duration but also had less severe symptoms than did those who used a gel that did not contain zinc.', 'Drug-company researchers experimenting with a nasal spray based on zinc have found that it has much the same effect on colds as the gel does.', 'To make them palatable, zinc lozenges generally contain other ingredients, such as citric acid, that can interfere with the chemical activity of zinc.', 'The mechanism by which zinc affects the viruses that cause the common cold has not been conclusively established.']