题目内容：

It is such a difficult problem___nobody can work out.为什么要用as而不是that

优质解答

如果是填that的话,组成such……that的结构,从句就要是完整的句子.
It is such a difficult problem___nobody can work it out.要有it
填as,此句是定语从句.as作为引导词在从句中做宾语.