all ( ) can be done must be done.

all ( ) can be done must be done. 合并句子all can be done.all must be done 用what或者who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 can be done been done. can be done was done could be done must have been done will All that can be done _______(be)done All that must be done----(do) there must something can be done something must be done must have been done/must be done all that can be done has been done