I can still remember that it is since the 1990’s we began to use cell phones the number

问题补充：

I can still remember that it is since the 1990’s, we began to use cell phones, the number of mobile users has greatly increased．

A. while; when

B. when; what

C. before; that

D. when; that

答案：

A. while; when

B. when; what

C. before; that

(答案→)D. when; that

解析：考查强调句式第一空是when引导定语从句，表示“我们在那个时候开始使用手机”，第二空是强调句式的引导词，被强调的部分是时间状语，因此D项正确。