Sales of CDs have greatly increased since the early 1990s when people to enjoy the adva

问题补充：

Sales of CDs have greatly increased since the early 1990s, when people to enjoy the advantages of this new technology.

A. begin

B. began

C. have begun

D. had begun

答案：

A. begin

(答案→)B. began

C. have begun

D. had begun

解析：此题考查since以及定语从句的时态照应。since加过去时间点，主句自然用完成时态。既然since跟的是过去的时间点，那么when指代的当然就是过去时间in the early 1990s，所以选过去式began。