In Cialdini, Reno, and Kallgren's First Study, the Independent Variable

In Cialdini, Reno, and Kallgren's first study, the independent variable was:

A) whether the participants saw a model picking up a piece of litter.
B) the amount of litter the participants encountered when entering the parking garage.
C) whether a parking garage or a mail box was used.
D) the nature of the flyer's message that was put the car's windshields.