When Harry's publishing company prints 800 books per day its total cost is $4000.00. Its total costs increase to $5000.00 when it produces 1300 books per day. The firm's total fixed costs are $1000.00 per day. The marginal cost of producing the 1300h book is
a. $4.00
b. $5.00
c. $1.25
d. $2.00