问题
选择题
It is believed to _____ over by sandstorms from AD 200 to AD 400.
A．gradually cover
B．be gradually being covered
C．have been gradually covered
D．be gradually covered
答案
答案：C
题目分析:句意：人们都认为它是在公元200年到公元400年期间被风沙逐渐覆盖的。固定句型：sb is believed to do “人们认为……”，相当于it is believed that +从句，故答案选C。