How Did the Stanford Prison Study Researchers Come to the Conclusion

How did the Stanford Prison Study researchers come to the conclusion that roles,and not individual personalities,were the main influence on the volunteers' behavior?

A) The volunteer "prison guards" were specifically instructed to respond brutally.
B) The "prisoners" were actually actors hired by the researchers.
C) The prisoners and prison guards were psychologically similar prior to the start of the experience.
D) The researchers actually believed that personality is more important than social roles.