Answer:
D. The Nash equilibrium is for Firm 1 and Firm 2 each to produce 10. 
Step-by-step explanation:
 Firm 2
 10 units 20 units
 10 units 30 / 50 /
Firm 1 30 35
 20 units 40 / 20 / 
 60 20
(firm 1 /
 firm 2)
Firm 1's dominant strategy would be to sell 10 units with an expected payoff outcome = 30 + 50 = 80
Firm 2's dominant strategy would be to sell 10 units with an expected payoff outcome = 30 + 60 = 90
Since both firms have the same dominant strategy (to produce 10 units), there is a Nash Equilibrium where both firms produce 10 units and each one earns 30.