Mark was a student at this university from 1999 to 2003, ____he studied very hard and was made Chairman of the Students’Union.
A. during which time                 B. for which time
C. during whose time                 D. by that time

还原成并列句则为：

Mark was a student at this university from 1999 to 2003, and during that time he studied very hard and was made Chairman of the Students’Union.

that为指示限定词，that time= the time he was a student at this university.

这个并列句改为主从复合句时，去掉并列连词and, 再用关系限定词which取代指示限定词that，则得到原句。

所以，非限制性定语从句并不是修饰from 1999 to 2003, 而是修饰主句的主系表结构。

这个句子结构类似于下句：

He stood by the window, from which place he could see the mountains on the horizon.

He stood by the window, and from that place he could see the mountains on the horizon.

that为指示限定词，that place = the place he stood.

所以，同理，这个非限制性定语从句并不是修饰名词window，而是修饰主句内容。

He stood by the window, from where he could see the mountains on the horizon.

He stood by the window, and from there he could see the mountains on the horizon.

所以，以上句子中的which均为关系限定词，而不是关系代词。后面需要名词time或place表示谓语动作发生的时间或地点。定语从句都是修饰主句内容的，不是修饰时间或地点名词的。

They stayed with me three weeks, ____they drank all the beer I had.
A. for which                          B. which time
C. during which time             D. during which

C为正确答案，D为错误。