Everything can be done has been done.请问这里can be done 用的是什么时态

Everything can be done has been done.请问这里can be done 用的是什么时态 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 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 can be done was done could be done must have been done will All that can be done __been done.请问该用has还是 have? America has never been about what can be done for what has been done can not be undone