all that can be done has been done (all that)can be done(has)been...

\u4e3b\u8c13\u4e00\u81f4\uff1a\uff08\u5e76\u8bf4\u660e\u539f\u56e0\uff09All that can be done___.A.has been done B.has done C.have done D.were done

1.A \u539f\u56e0\uff1aall that=what.\u800cwhat\u505a\u4e3b\u8bed\u89c6\u4f5c\u5355\u6570\uff0c\u56e0\u6b64\u6392\u9664CD\u3002\u800c\u4e8b\u60c5\u662f\u88ab\u505a\uff0c\u5e94\u8be5\u7528\u88ab\u52a8\u8bed\u6001\uff0c\u56e0\u6b64\u9009A\u3002\u53e5\u610f:\u6240\u6709\u80fd\u88ab\u505a\u7684\u4e8b\u90fd\u5df2\u7ecf\u88ab\u505a\u5b8c\u4e86\u3002
2.C \u539f\u56e0\uff1avery few\u610f\u601d\u662f\u6781\u5c11\u4eba\uff0c\u5176\u540e\u8c13\u8bed\u52a8\u8bcd\u4e0d\u9700\u8981\u7528\u7b2c\u4e09\u4eba\u79f0\u5355\u6570\u5f62\u5f0f\uff0c\u6392\u9664AD\u3002\u7531\u4e8e\u6574\u53e5\u8bdd\u662f\u5728\u9648\u8ff0\u5ba2\u89c2\u4e8b\u5b9e\uff0c\u56e0\u6b64\u76f4\u63a5\u7528\u4e00\u822c\u73b0\u5728\u65f6\u5373\u53ef\uff0cB\u9009\u9879\u4e2d\uff0cknow\u7684\u610f\u601d\u5728\u8fd9\u91cc\u662f\u201c\u77e5\u9053\u201d\uff0c\u662f\u77ac\u95f4\u52a8\u8bcd\uff0c\u800c\u77ac\u95f4\u52a8\u8bcd\u4e0d\u80fd\u7528\u4e8e\u73b0\u5728\u8fdb\u884c\u65f6\u6001\u4e2d\uff0cB\u9519\u3002\u53e5\u610f\uff1a\u5728\u8fd9\u4e2a\u9547\u4e0a\u5f88\u5c11\u4eba\u77e5\u9053\u4ed6\u7684\u4f4f\u5740\u3002

all that = what
what can be done\u4f5c\u4e3b\u8bed

这里你或许没理解，All that 这种结构后面是跟动词的单数形式的，不是复数形式,你从中文来看好像是所有东西都使用了，是复数的意思，但是在英文结构上你没看到任何有复数的名词形式，你是用动词单数的。但是，比如你用了All methods/approaches(方法）that can be done则后面用have been done，因为前面采用了名词的复数形式。
另外比如everything, nothing, anyhing, anything, nobody, anybody, somebody, someone, everyone等不定代词，后面也是用动词的单数形式的

all修饰that,没有出现复数形式，所以用has

all在这里表示一个整体的概念，所以用单数has.
for example:
1 all is right.一切顺利。
2 ten yuan is enough. 十元够了。

能做的都已经做了
all作代词，表全体、一切、全部，则动词一般都是用单三

因为主语是代词all，强调一个整体

扩展阅读：all that i can do is ... many a student ... all that can be done ... one can see that ... and all that i can see ... that s all thank you ... tom is the only one ... more than one worker ... that we can baby ...