Class was over,but the difficult physics problem _____ unsolved.
A.left B.kept C.taken D.remained.
学生不知道B、D选项如何选，kept和remained都有“保持”的意思，都可以做系动词，unsolved经查证也可以做形容词。是不是这两个词的意思上有微妙的差别呢？盼解答谢谢！