那个知道All that__must be done 填什么?

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 ( ) can be done must be done. all that must ___ 填to be done 为什么不能填 be done.非得接不定式么? all that can be done been done. 合并句子all can be done.all must be done All that can be done _______(be)done all the things should be done have been done already all that can be done has been done Has all that can be done been done? all that can be done has been done. All that must be done----(do)