all that can be done has been done 为什么要用has,不用have

题目

all that can be done has been done 为什么要用has,不用have

答案

这句话的主干：All has been done .
that can be done(定语从句)
主句主语all 在此指代“所有事”= everything,是不可数概念,故谓语用单数形式：has