it is the most difficult question (for us) to solve.
当中省略了for sb ,很多这样的用法的呢.省略了for sb ,这种用法都不要用被动吗?能给个语法支持吗?是的，因为用上了for sb　的话，就用动词不定式了呢。而hard to solve, difficult to solve 语法上没错