有這樣一道單選題:Class was over,but the difficult physics problem _____ unsolved.
A.left B.kept C.taken D.remained.
學生不知道B、D選項如何選,kept和remained都有“保持”的意思,都可以做系動詞,unsolved經查證也可以做形容詞。是不是這兩個詞的意思上有微妙的差別呢?盼老師解答謝謝!