Attempts to Guard Against Harm to the Participants in the Simulated

Attempts to guard against harm to the participants in the simulated prison were accomplished by all but which of the following?

A) making certain that all participants were psychologically healthy
B) insisting that those who were assigned the guard role would not use physical force against those assigned to the role of prisoner
C) any participant who showed distress was released from the simulation early
D) telling the participants in advance about all the potential harm