Has all that can be done been done?

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 can be done been done. all that can be done has been done 为什么要用has,不用have all that can be done has been done.中的that为 Everything that can be done has been done,___? has everything that can be done been done句子里的that能不能省略,为什么?如 nothing that can be done has been done 可省that吗 在all that should be done has been done用了has,但在all that shoul 主谓一致：（并说明原因）All that can be done___.A.has been done B.has do I don't think anything that has been done can be done.怎么翻译