he referred to in his article was unknown to the general reader. A. ThatB. WhatC. Whether

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(答案→)A. That
B. What
C. Whether
D. Where
解析：主要测试主语从句。句子     he referred to in his article在复合句中充当主语，同时该句子中缺少介词to的宾语，双重作用。答案一目了然。