参考答案：A

解析：[分析] 细节题型。 从第二段第二句Informal mechanisms include expressions of disapproval by significant others and withholding of positive rewards for disapproved behavior．我们可以排除B、C两项。根据第三句Most people internalize norms in the course of socialization．的信息可判断D项不是正确答案。第五句When informal sanctions fail, formal agents of social control may be called upon．我们可知本句前面的内容指的都是 informal的内容可以排除D项。