Nash Equilibrium Is When

Nash equilibrium is when

A) no single actor can be made better off without at the same time making another actor worse off.
B) at least one actor can be made better off without at the same time making another actor worse off.
C) neither player has an incentive to change strategies unilaterally.
D) both players have the advantage to change strategies unilaterally.
E) one player has the advantage to change strategies unilaterally but the other player doesn't.