Which of the Following Is True for a Nash Equilibrium

Multiple Choice

Which of the following is true for a Nash equilibrium of a two-player game?

A) The joint payoffs of the two players are highest compared to other strategy pairs.
B) Given another player's strategy stipulated in that Nash equilibrium, a player cannot improve his welfare by changing his strategy.
C) A Nash equilibrium is always unique in real-world problems.
D) Given another player's strategy stipulated in that Nash equilibrium, a player cannot improve his welfare by changing his strategy, and a Nash equilibrium is always unique in real-world problems.