all that must ___ 填to be done 为什么不能填 be done.非得接不定式么?

all that must ___ 填to be done 为什么不能填 be done.非得接不定式么? All that must be done----(do) all ( ) can be done must be done. all that can be done been done. 合并句子all can be done.all must be done All can be done. All must be done.合并为一个句子怎么写? 为什么? All that must be done ____(do).答案是have been done,为什么是这个答案?这句 be to be done可以改成must be done 那个知道All that__must be done 填什么? 那个知道All that_must be done填什么? All that can be done _______(be)done something must be done