It was not until midnightthey reached the camp site.A that B when C while D as

问题补充：

It was not until midnightthey reached the camp site.

A,that B,when C,while D,as

答案：

解析：通过分析句子结构，可知这是一个强调句。因为去掉it was和that，剩下的是一个完整的句子，所以是强调句型。