If the Prisoners' Dilemma Is Repeated Over and Over Again

If the Prisoners' Dilemma is repeated over and over again with the same two players having an indefinite time horizon,

A) the unique equilibrium is to play the Nash equilibrium of Rat each period.
B) players can cooperate on Silent if they are not too patient.
C) players can cooperate on Silent if they are patient enough.
D) players can only cooperate on Silent in the initial stages of the game.