We should do what we can ( ) those in trouble .

问题描述:

We should do what we can ( ) those in trouble .
A, to help
B. heip
C. helping
D, will heip
答案是A ,但是我觉得can后面应该是动词原型呀,怎么解释

是宾语从句修饰do
是做目的状语
原来的基本句型是 We should do
do后面的宾语是个从句 即
然后do的目的是to help those in trouble