A stationery firm produces and sells staplers. Last year, it produced 5000 staplers and sold each stapler for $10. In producing the 5000 staplers, it incurred variable costs of $25,000 and a total cost of $45,000.

In producing the 5000 staplers, what was the firm's average fixed cost?

a. $3

b. $4

c. $5

d. $6