Rareobjectsarebeautifullydisplayedinglasscases____themfreefromdust.
A.whicharekeeping,
B.whichkeep.
C.whokeep
D.andkept.
这句话中为什么选B,不选D,为什么只能用定语从句,不能用并列句,区别是什么,它们在一个句子里,谢谢