题目内容：

He is often heard _______ the English song.
A.to sing B.sing C.to have sung D.to be singing
2.we don't know _____
A.to do B.to be done C.what to do D.what to be doing
我都知道答案,但是不明白这是什么原理.只句的语感是这样.希望大家能说一下原因.