OG12 85比较补出助动词，还是不明白

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 flurished 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

选A, 为什么不补出后面的助动词呢，是不是因为这里的at the same time as就相当于more than的作用，这样看at the same time as的前后比较的就都是civilization了？