It was not until mid-night they reached the camp site.
A. that
B. when
C. while
D. before
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(答案→)A. that
B. when
C. while
D. before
解析：考查强调句型。本题强调的是时间状语not until mid-night；句意：直到半夜，他们才到达宿营地。故A正确。要注意如果不使用强调句型，Not until mid-night did they reach the camp site。要使用部分倒装句。