All I could do is to send him to school. A.which B.what C.that D.the thing

All I could do is to send him to school. A.which B.what C.that D.the thing
All I could do is to send him to school. A.which B.what C.that D.the thing

All I could do is to send him to school. A.which B.what C.that D.the thing
C that.这是一个定语从句：All I could do is that to send him to school.其中,All是这个定语从句的先行词.在定语从句中如果先行词是All的话,关系代词就必须用that.

B.what

C that 同意楼上观点