Choice A, the best answer, uses the simple past tense [U]flourished[/U] to describe civilizations existing simultaneously in the past.
Choice B wrongly uses the past perfect "had flourished"; past perfect tense indicates action that was completed prior to some other event described in the simple past tense: for example, "Mayan civilization had ceased to exist by the time Europeans first reached the Americas."
Choice C lacks [U]as[/U] after [U]time[/U].
In choices C, D, and E, the plural pronoun [U]those[/U] has no plural noun to which it can refer.
In C, [U]had[/U] signals the incorrect past perfect; [U]did [/U]in D and [U]were[/U] in E are awkward and unnecessary.
D and E also incorrectly use the present participle [U]flourishing[/U] where [U]that flourished [/U]is needed.
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