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.

1. Whether zinc is effective only against colds, or also has an effect on other virally caused diseases题外话

2. Whether there are remedies that do not contain zinc but that, when taken orally, can reduce the duration of colds题外话

3. Whether people who frequently catch colds have a zinc deficiency
4. Whether either the zinc gel or the lozenges contain ingredients that have an impact on the activity of the zinc
5. Whether the zinc gel has an effect on the severity of cold symptoms, as well as on their duration

大牛解释一下这题

答案是D吧

D提出可一个其它可能的原因影响效果，就是是否这两个东西里面有什么物质或者元素会影响zinc的活性。如果有

那就是活性倒置的而不是因为鼻子的愿意。如果没有，那就是排除他因，加强了原文结论。