—Did Alan see his old friends yesterday?

—Yes, he did. He     his old friends for several years.

A．didn’t see

B．wouldn’t see

C．hasn’t seen

D．hadn’t seen

在产品生命周期中（），销售促进最有效。

A.引入期和衰退期

B.引入期和成长期

C.成长期和成熟期

D.成熟期和衰退期