og 11 求教1题, AS 的用法

OG 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

这道题从指代里面也可以得出正确答案A, 但是我对里面的AS难以理解,AS后面要跟句子,为什么这里不跟句子阿 ,而且D和E的解释中在句子意思已经明确的情况下, DID以及WERE可以省略,难道AS后面的动词还可以省略? 请指教,谢谢了

 这整个是一个从句，
                that flourished ( verb.) at the same time as the civilizations  in the Nile delta and the river valleys of the Tigris and Euphrates ，

 the same ...as, 是句式，不是单一的as

 the same ...as, 是句式，不是单一的as

那AS后面有没有省略现象,或者说这种省略在GMAT的考试中是不是不对的