Ask Li Mei,he ------know the answer .

选C,根据后面的动词know,用原形,说明前面必须加个情态动词,A会导致两个动词重复(be 和know); B选项是个副词,D must 表示一定,跟句子表达的语气有差异.maybe该怎么用，副词不是修饰动词吗？是修饰动词,如果用副词来修饰的话 ,know必须改成第三人称单数,knowsmaybe可以放句子前面,也可以放在动词前面.Maybe, he knows the answer.=== He maybe knows the answer.主要是跟may be 的区别, 写个句子让你比较下大概就清楚了.He maybe (is) a teacher.== He may be a teacher.注意第一个句子里的is