Why Did the Zimbardo Prison Study Have to Be Stopped

Why did the Zimbardo prison study have to be stopped before the desired end date?

A) The participants took on the roles they were given so seriously, it was apparent that harm was being done to those in the prisoner role.
B) The participants who played the guards were unable to maintain control over the prison population.
C) The participants were unable to keep their commitments due to school work.
D) The participants in the study were unable to switch roles from student to prisoner and prison guard.