All can be done has been done.A. whichB. thatC. /D. what

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A. which
(答案→)B. that
C. /
D. what
解析：考查定语从句，先行词是all，定语从句中缺少主语，用关系代词，当先行词被最高级或序数词，不定代词修饰时，定语从关系代词用that，不用which，选A。句意是：所有能做的都已经做了。选B。