At the current usage level of a single variable input (used in conjunction with fixed quantities of other inputs), a firm finds that the average productivity of the variable input is 4 units of output per unit of input. The price of the variable input is $16. At its current level of output the firm's average fixed cost is $20 per unit of output. The average variable cost of production is therefore :
A. $4 per unit of output
B. $9 per unit of output
C. $16 per unit of output
D. $64 per unit of output
Explain your answer.