答案：D

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。