问题补充：
Sales of CDs have greatly increased since the early 1990s, when peopleto 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：本题考时态和句子结构。句中when是关系副词，引导定语从句，修饰the early 1990s。从句的意思是：当时（指in the early 1990s）人们开始喜欢这一新科技带来的好处。讲的是过去发生的事，与现在无关，排除A、C。从句中有表示过去具体时间的状语（when相当于in the early 1990s），不能用过去完成时。
