Question

The commercial city, Loulan, used to be a stopping point on the silk road between the West and the East, ______ by sandstorms from AD200 to AD 400.

A. which is believed to be covered over

B. as is believed to have been covered over

C. it is believed to be covered over

D. which is believed to have been covered over

Answer

D

【解析】

考查定语从句。句意：商业城市楼兰曾是东西方丝绸之路上的一个中转站，据信，从公元200年到公元400年，这里曾被沙尘暴覆盖。______ by sandstorms from AD200 to AD 400.是一个定语从句，先行词是Loulan，排除C；as引导的非限制性定语从句，先行词不可是一个词，必须是整个主句或主句的部分内容，排除B；楼兰是被覆盖，并且事情是过去发生的；故用动词不定式的完成式，故选D。