His friends did ____ they could ______ him out of difficulty

问题描述:

His friends did ____ they could ______ him out of difficulty
1.all what;to help 2.all that ;help
3.all;to help 4.what;help
答案是.3但是为什么不是His friends did all they could do to help him out of difficulty呢,2.为什么不是同位语从句

原句没有do用的是省略的用法,就算do 不省略,do后面依然缺乏宾语.同位语从句是不会缺成分的.而定语从句that引导的时候就可以做宾语.that这里已经省略了.