问题
选择题
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
B
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。