Some students even have no idea of they are studying so they waste much time playing.A. w

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。