I don't know what _____.A.do B.doing C.did D.to do

I don't know what _____.A.do B.doing C.did D.to do
I don't know what _____.A.do B.doing C.did D.to do

I don't know what _____.A.do B.doing C.did D.to do
D
what 引导了一个宾语从句
to do 是不定式
其实这种结构很多的,多多锻炼语感吧

D 动词不定式