题干
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 |
答案(点此获取答案解析）
