Mark was a student at this university from 1999 to 2003,____

Mark was a student at this university from 1999 to 2003,______ he studied very hard and was made C
for which time
不是也表示一段时间内吗
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 whose time B.during which time C.for which time D.by that time
for which time
不是也表示一段时间内为什么不能用？