问题补充：
advertisements are of great help, I don’t think we should entirely rely on them.
A. Since
B. While
C. Because
D. As
答案：
A. Since
(答案→)B. While
C. Because
D. As
解析：考查连词：A. Since自从，既然 B. While 虽然，当.. 时候，然而Because因为 D. As因为，当…时候，句意：虽然广告很有用，但我认为不能完全依赖它们。选B。
