Question

the book was so interesting that he had read it for three hours___he realized it.

the book was so interesting that he had read it for three hours___he realized it.
A.when B.before C.if D.after
选什么？为什么这么选？

Answer 1

选b,说的是这本书是那么有趣以至于他读了三个小时了自己还没觉察出来。就是在他觉察到之前已经读了三个小时了。三个小时在前，觉察在后，正是before的在什么之前的用法和意义。
读三个小时之后才觉察
觉察之前有三个小时
诸如此类的就用before

Answer 2

A。你查一下when的用法，主句是过去完成时，when引导的从句为一般过去式。表示当他意识到时，已经.....