He is quite strange, for everything he does is opposite to is considered normal behavior

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A. whether
B. that
(答案→)C. what
D. which
解析：空格出缺少连词，连词引导的是宾语从句，做介词to的宾语。whether ，that在宾语从句中不做成分，只起连接的作用，故排除AB；what在宾语从句中可做主语或宾语，起连接的作用；which在宾语从句中可做定语，故排除D，所以应选C。