9. Which of the following statements is true?

A)A subgame perfect equilibrium need not be a Nash equilibrium.

B)subgame perfection requires players to always play optimally only on the equilibrium path of play in any game of perfect or imperfect information.

C)A subgame of a game cannot include the game itself.

D)Subgame perfection requires players to use strategies that constitute a best response or a mutual best response in every subgame of the larger game.