They tried their best to do __________ help us. A. that they could B) all that they could

解析：do是及物动词,所以后面接的是宾语从句.【what they could 】【to】可以分为两部分来理解what they could（do）其实这里省略了动词【do】,what 做do的宾语 .what they could do可以理解成一个名词.就是做前面那...真不是很理解，could不是情态动词吗，加动词原形？对啊 ，could是情态动词，后面接动词原形，上面不是说其实could后面还有do省略了。what they could do尽他们所能。这是宾语从句。what 做do的宾语。你就把 what they could do当成一个名词就行,就用sth来代替。They tried their best to do 【sth】to help us不理解的话再详细点问再详细点吧，灰常感谢！！【what】宾语【 they】主语【could （do）】谓语，可以颠倒下语序。They could do what to。。。。to 表示目的。再不明白我也没办法了你仔细看看回答吧