Why is it, in the study by Cialdini and his colleagues (1990), that a single piece of trash is more effective at reducing litter than a perfectly clean landscape?

a. It signifies the injunctive norm of not littering.
b. It is a huge eyesore that prompts people to pick up after themselves and others.
c. It sticks out in a clean area and reminds others not to be careless.
d. It creates a descriptive norm of some allowable litter.

Answer: C