nearly all behavior is learned behavior is a basic assumption of social scientists.
A. /
B. That
C. It is acceptable
D. When
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A. /
(答案→)B. That
C. It is acceptable
D. When
解析：根据句子的结构分析，句中的nearly all behavior is learned behavior是一个完整的句子在充当主语，称为主语从句。that引导主语从句只起连接作用，没有实际含义，但不能省略。