Heisoftenheard_______theEnglishsong.
A.tosingB.singC.tohavesungD.tobesinging
2.wedon'tknow_____
A.todoB.tobedoneC.whattodoD.whattobedoing
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.