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 timeB.for which timeC.during whose timeD.by that time在线课程A
解析：
解析：from 1999 to 2003是表示一个时间段，故在用介词+关系代词引导定语从句时，介词需用during表示“在……期间”，在这一结构中，所用的关系代词只有两个；
指人时用whom，指物时用which，故不能选C．命题立意：考查非限制性定语从句，此题难度较大，考生需牢牢掌握基础知识，才能顺利选题．