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
网友回答
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。