请教关于OG10-80的一个疑问

80. Salt deposits and moisture threaten to destroy the Mohenjo-Daro excavation in Pakistan, the site of an ancient civilization that flourished at the same time as the civilizations in the Nile delta and the river valleys of the Tigris and Euphrates.

(A)  that flourished at the same time as the civilizations

(B)  that had flourished at the same time as had the civilizations

(C)  that flourished at the same time those had

(D)  flourishing at the same time as those did

(E)   flourishing at the same time as those were

这个题目的答案给出的是A。我的疑问是为什么A的结构不需要像B一样，在the civilizations之前加上did？

如果这个是一个正常句式，应该是这样的形式吧？：

An ancient civilization flourished at the same time as （did）the civilizations  in the Nile delta.

在这种情况下，后面的动词不需要补出来吗？

这个疑问一直都没搞太清楚，请NN们帮忙解答疑惑，多谢了！

支持！！！为什么没人来回答啊？？？