advertisements are of great help I don’t think we should entirely rely on them.A. SinceB.

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：

这题考查连词的用法，Since自从，While虽然，当…时候，Because 因为， As因为，当..时候，根据句意：虽然广告很有用，我想我们不该完全依赖它们。选B。