After a year-long drought, the city of Pine Bluffs has banned all lawn sprinkling. Many residents believe, however, that continued watering of their own lawn will have little effect on total water reserves. Consequently, there is a disastrous drain on city water reserves caused by widespread illegal sprinkling. This incident best illustrates the dynamics of

A) a reciprocity norm.
B) social traps.
C) the bystander effect.
D) diffusion of responsibility