Our maths teacher set so difficult an examination problem ______ none of us worked out.
A.which
B.that
C.as
D.so that
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C解析试题分析：句意：我们的数学老师出了一个这么难的考试题，我们中没人能做出这个题来。这里是定语从句，so……as，as引导的是定语从句，as在从句中宾语，故选C。考点：考查定语从句的用法。点评：本题难度适中。定语从句是高中阶段必须掌握的语法项目，考生容易误选AB，这里是特殊的固定搭配，不能用that或which，在结果状语从句中that不作成分，故选C。即学即练：I saw some trees，the leaves of ______ were black with disease.
A. that
B. which
C. it
D. what解析：B。考查非限制性定语从句的介词+which结构。the leaves of which 相当于 whose leaves。