You Ate the Last of Your Roommate's Cereal

You ate the last of your roommate's cereal. When she asks "did you finish the cereal or did I?" you lie and say you didn't eat it. When she then asks you to stop by the store to pick up some more you agree. Your behavior is best explained by

A) the need to reduce private guilt.
B) the feel good-do good effect.
C) the social exchange theory.
D) the reciprocity norm.