请看这道高考题：
We are living in an age ______ many things are done on computer.(2003全国春季卷)
A. which B. that C. whose D. when
解析：本题的先行词是an age，表示“时代、年代”，从句是被动语态，句子结构是完整的，故选D。也就是说，当 age 在定语从句中充当时间状语时用 when，如果作主语、宾语、表语，则用 which, that 来引导（宾语时可以省略引导词）。请比较下面的句子：
The 21st century is an age that urgently need characteristic talents. 21世纪是个性人才急需的时代。
An age which is called Dark not because the light fails to shine, but because people refuse to see it. 一个年代之所以被称为黑暗,倒不是因为光线照耀不到,而是因为人们宁作睁眼瞎子。