One Major Conclusion from the Littering Studies by Cialdini, Reno

One major conclusion from the littering studies by Cialdini, Reno, and Kallgren was:

A) observing a person littering is the most important variable in explaining littering.
B) seeing litter at an amusement park influenced people more than seeing it on hospital grounds.
C) reading a sign that warned people not to litter influenced them the most.
D) realizing the norm in the environment is not to litter was the most important variable.