the teacher gave us so difficult a problem as most of us can't work out这里为什么要用as而不用that呢?
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that后要加从句（完整句子）,as再这里相当于which,指代problem
用that的话从句中要有it做宾语
即The teacher gave us so difficult a problem as most of us can't work out＝The teacher gave us so difficult a problem that most of us can't work it out