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。