问题补充：
It was not until midnight they reached the camp site.
A. that
B. when
C. while
D. as
答案：
(答案→)A. that
B. when
C. while
D. as
解析：考查强调句型。It is/was 被强调成分that/who其它成分；这是强调句型的基本结构。强调句型最大的特点就是把it is/was…that/who去掉，句子仍然成立。本题考查的是时间状语not until midnight。句意：直到半夜他们才到营地。
