nearly all behavior is learned behavior is a basic assumption has been put forward by s

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A. What; that
B. That; what
C. What; what
(答案→)D. That; that
解析：D　句意：几乎所有的行为都是学来的行为是社会学家提出的基本设想。“    nearly all behavior is learned behavior”为主语从句，该从句不缺任何成分，故用that引导；“    has been put forward by social scientists”为定语从句，先行词为assumption，将先行词代入定语从句后为：The assumption has been put forward by social scientists.由此可见先行词在定语从句中作主语，故用that引导。