21. The inverse market demand is given by p(y)=26y. Suppose there are only two firms and both of them have the cost function ci(yi)= 2yi.The firms are playing the infinitely repeated 7 Cournot game.For Grim Trigger Staregy to be a Subgame Perfect Nash Equilibrium of this game, the discount factor must be greater than (Round to the closest number) (a) 0.50 (b) 0.53. (c) 0.57. (d) 0.6