For Firm A,when 4 Units of Labor,the Only Variable Input

For Firm A,when 4 units of labor,the only variable input into the production process,are hired,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 labor is 5 units?

A) $2.00
B) $20.00
C) $20.80
D) $22.80