nearly all behavior is learned behavior is a basic assumption of social scientists.A. /B.

问题补充：

nearly all behavior is learned behavior is a basic assumption of social scientists.

A. /

B. That

C. It is acceptable

D. When

答案：

A. /

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。