Salt deposits andmoisture threaten to destroy the Mohenjo-Daro excavation in Pakistan, the siteof an ancient civilization that flourished atthe same time as (did)the civilizations in the Nile delta andthe river valleys of the Tigris and Eupgrates. 定语从句"that flourished atthe same time as (did)the civilizations",其中的did可以存在么?类似于这样的do/does/助动词,他们是必须存在的么?何时可以省略?