Suppose that for a particular firm the only variable input into the production process is labor and that output equals zero when no workers are hired. In addition, suppose that when the firm hires 4 workers, the firm produces 50 units of output. If the fixed cost of production is $4, the variable cost per unit of labor is $20, and the marginal product of labor for the fifth unit of labor is 2, what is the average total cost of production when the firm hires 5 workers? a. $2.00 b. $4.00 c. $5.00 d. $20.00