Can you explain this problem

Consider the U.S.-U.S.S.R. arms race of the 1950s in which each country could build nuclear missiles or refrain from building them. The payoffs to these strategies resemble the following normal-form game: U.S.S.R. Refrain Build Refrain (5,5) U.S. (2,4) Build (4,2) (3,3) (a) List the game's players and strategies. (b) Find the pure strategy Nash equilibrium/ equilibria of this game. Hint: Recall that the equilibrium is given in terms of strategies, and not payoffs. (c) Find the mixed strategy Nash equilibrium of this game. (d) What has to happen to this game such that (Refrain, Refrain) is the only Nash equilibrium? Explain your answer. Hint: You can provide only a sufficient condition