问题补充:
Some students even have no idea of they are studying, so they waste much time playing.
A. what
B. when
C. why
D. where
答案:
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。