A firm's production function is given by Q = KL. The marginal products of labor and capital are, respectively, MPL = K and MPK = L. The wage rate of labor is w = $10 and the rental rate of capital is r = $20. The firm wants to produce 1,800 units of output in the most efficient way possible. How much does the firm spend?
A) $2,000
B) $1,300
C) $1,200
D) $1,100