The Payoff Matrix Below Shows the Payoffs for Firm a and Firm

The payoff matrix below shows the payoffs for Firm A and Firm B,each of whom can either "cooperate" or "cheat." The numbers in parentheses are (payoff for A,payoff for B) .
Firm B
Firm A TABLE 11-2
-Refer to Table 11-2.If Firm A is indifferent between cheating or cooperating when Firm B chooses to cooperate,x must be equal to

A) 0.
B) 10.
C) 20.
D) 30.
E) 40.