Two people facing the prisoners’ dilemma may cooperate if

a. they recognize that the Nash equilibrium is worse for both people than the cooperative equilibrium.

b. they will play the game repeatedly and expect noncooperation to be met with future retaliation.

c. each chooses the strategy that is best for herself, given what the other person is doing.

d. each realizes that the strategy she chooses is not known to the other until the outcome is realize