Some students even have no idea ofthey are studying so they are aimless most of the time.

问题补充：

Some students even have no idea ofthey are studying, so they are aimless most of the time.

A. who

B. when

C. where

D. why

答案：

A. who

B. when

C. where

D. why

解析：选D。考查宾语从句。句意: 一些学生甚至不知道他们为什么学习, 所以他们大部分时间漫无目的。根据语境aimless推知“不知道学习的原因”, 故答案D合适。