In a study on littering behavior, participants were observed in a parking lot scenario. What action increased the likelihood of individuals dropping a flier on the ground?
a) Witnessing someone else picking up litter in a clean parking lot.
b) Witnessing someone else littering in a clean parking lot.
c) Witnessing someone else littering in a parking lot filled with litter.
d) Witnessing someone else picking up litter in a parking lot filled with litter.