In the Classic Social Psychology Experiment Known as the Stanford

In the classic social psychology experiment known as the Stanford Prison Experiment,how was group selection determined?

A) the most sadistic participants were assigned the roles of guards.
B) the participants who were thought to be "helpless" types were assigned the roles of prisoners.
C) the participants were assigned roles that were the opposite of their natural inclinations.
D) the participants were assigned roles through random procedures.