They tried their best to do __________ help us. A. that they could B) all that they couldThey tried their best to do __________ help us. A. that they couldB) all that they could C. what they could to D. what they could请高人解释下为什么

They tried their best to do __________ help us. A. that they could B) all that they couldThey tried their best to do __________ help us. A. that they couldB) all that they could C. what they could to D. what they could请高人解释下为什么
They tried their best to do __________ help us. A. that they could B) all that they could
They tried their best to do __________ help us. A. that they could
B) all that they could C. what they could to D. what they could
请高人解释下为什么是C呢?

They tried their best to do __________ help us. A. that they could B) all that they couldThey tried their best to do __________ help us. A. that they couldB) all that they could C. what they could to D. what they could请高人解释下为什么
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.就是做前面那个do的宾语.
动词不定式【to】表示目的,为了、、、
下面逐一分析各个选项：
A、that 引导宾语从句的时候,只起到连接的作用,不做句子成分,如刚才所说.could后面省略了do,所以do要接宾语,that不符合.批出A
B、all that=what,但是选项中缺少to做目的
D 同样缺少to 不赘言.
无疑后请采纳.
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