Some students even have no idea of they are studying, so they waste much time playing.
A. what
B. when
C. why
D. where
网友回答
A. what
B. when
(答案→)C. why
D. where
解析：考查宾语从句：句意：一些学生甚至不知道为什么学习，因此浪费了很多时间玩。可知用why连接宾语从句，what在宾语从句中做主宾表，这个宾语从句中不缺主宾表，BD选项意思不符合。选C。