I'll do ____help you.
A.what i can B.all i can C.all that i can D.all i can do to 答案为D，为什么，请一一分析每个答案。
参考答案:因为只有D后面接了不定式to
这句的谓语是will do，后面如果再出现别的动词，一定要加动词不定式的。
I'll do all I can do to help you.
all是宾语，I can do是宾语补足语,其实句子的主干就是I'll do all to help you.