You Were Walking Down the Street with Your Friend Sabrina

You were walking down the street with your friend Sabrina when she stopped to ask a child why he was crying.The boy said that he was lost,so Sabrina took time out of her busy day to help the child find his way home.Sabrina's behavior reflects

A) the reciprocity norm.
B) egoism.
C) the social-exchange theory.
D) altruism.