语法填空,The concert is fully expected to be put off until another.为什么用to be put

这是 expect the concert to be put off 的被动语态形式
the concert（音乐会） to be put off （被推迟）是个复合宾语（宾语+宾补）,
可见 to be put off 的逻辑主语是the concert ,由于表被动,所以用不定式的被动式