1.
(2019高一下·徐州期中)
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
