How Can Parties Who Find Themselves in a Prisoner's Dilemma

How can parties who find themselves in a prisoner's dilemma situation avoid the undesired outcome and cooperate with each other?

A) one oligopoly can physically beat up another oligopoly
B) by seeking alternatives to create pressure for members to keep output up and prices up
C) find effective ways to penalize firms who do not cooperate
D) sign legally enforceable contracts setting out their mutual agreement to act like a monopoly