25．Helen ran out of the classroom in tears, but no one knew ______ she was so sad.
A
when
B
whether
C
that
D
why
句子的意思是，她哭着冲出教室，但是没有人知道她为什么那么伤心。所以选D。
考查状语从句的引导词。
错误理解两个分句的逻辑关系而误选其他选项。