问题详情：
As a stopping point on the famous Silk Road, Loulan is believed __________ by sandstorms from AD 200 to AD 400.
A．to be gradually covered over B．to gradually cover over
C．to have been gradually covered over D．to gradually have covered over
【回答】
C
【详解】
考查固定句式。句意：大家认为作为丝绸之路的中转点而出名的楼兰，是在公元200至400年间慢慢地被沙尘暴覆盖的。Sth./Sb. is believed to have done…“某人/某物据相信已经做了”因为这件事已经发生，先于believe发生，所以用不定式的完成式，副词置于助动词后，故选C。
知识点：固定句型
题型：选择题
