高三英语Nothing.I was just thinking about the problem _ tomorrow.A.to be discussed B.being discus

选A.
相信不选B C的理由你明白的了,那么为何不能选D呢?因为你选了D,那么这个句子就变I was just thinking about
the problem will be discussed tomorrow.有两个主语（I 和 the problem）,两个动词（think 和 be discussed）,相当于有两个句子了,专业点来讲就是有两个主谓宾结构,这样的句子就是错句.
如果非要用will be discissed,那么可以用连接词 that 或者 which 将后面的句子变成从句,即 I was just thinking about
the problem that/which will be discussed tomorrow.这样句子就正确了.