All that must be done _________.(do的适当形式）

All that must be done _________.(do的适当形式） All that must be done----(do) All that must be done ____(do).答案是have been done,为什么是这个答案?这句 all ( ) can be done must be done. 合并句子all can be done.all must be done all that can be done been done. all that must ___ 填to be done 为什么不能填 be done.非得接不定式么? BUT I WANT I MUST I DO APOLOGIZE FOR ALL THAT I'VE DONE All that can be done _______(be)done 主谓一致：（并说明原因）All that can be done___.A.has been done B.has do Do you like _________（they）,too?（用所给单词的适当形式填空） 希望尽快解决~适当形式填空:1.All flight must be _______(cancel) because o