一道高中英语语法题The lecturer was a tall thin man with white hairThe

一道高中英语语法题
The lecturer was a tall thin man with white hair
Then it_______Dr.Fell because he is short and fat.It_____Dr.Jones,he is thin
A.cant' be...must be B.couldn't have been...might have been
C.must be...can't be D.can't have been...must have been
书上正确答案应该选D,为什么不能选B?
上句写的是was,下句不该用过去时could吗?而且lecturer是thin的,但thin的人又不一定就是lecturer咯,只是可能是lecturer,所以我用might have been,最终选了B.
大家帮我看看,我分析中的错误,为什么我分析出来B对的,人家正确答案却选D,