A competitive firm is using only labour and capital as inputs, and it maximizes its profits in the long run. You know that the firm pays an hourly wage of $20 and its hourly cost of a unit of capital is $15. if this firm's marginal product of labour is 10 units per hour, then its marginal product of capital per hour must be in the long run. OA 7.5 OB. 30 O C. 13.33 OD. 6.67