Why Does a Prisoner's Dilemma Lead to a Non-Cooperative Equilibrium

Why does a prisoner's dilemma lead to a non-cooperative equilibrium?

A) Because each player agreed before the game started to minimise the harm that he can inflict on the other players.
B) Because each player is uncertain how other players will play the game.
C) Because players must choose from a limited number of non-dominant strategies.
D) Because each rational player has a dominant strategy to play a certain way regardless of what other players do.