问题描述：
求助一句英语句子的语法
it's believed by many people to have been gradually from AD200 to AD500.
这是我们英语课本上的一句句子 to have been 这里的to 跟哪个单词
covered over by sandstorms 这里的 over 是什么用法?
it's believed by many people to have been gradually from AD200 to AD500.
这是我们英语课本上的一句句子 to have been 这里的to 跟哪个单词
covered over by sandstorms 这里的 over 是什么用法?
问题解答：
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