Reno and colleagues put fliers under the windshield wipers of cars in a parking lot, to see whether people returning to their cars would litter by dropping the flier on the ground. Results from this study showed that people were most likely to drop the flier on the ground if they:
a) saw someone else pick up litter in a parking lot clean of litter.
b) saw someone else litter in a parking lot clean of litter.
c) saw someone else litter in a parking lot filled with litter.
d) saw someone else pick up litter in a parking lot filled with litter.