Buenos Company has two divisions, Division X and Division Y. Division X has a production capacity of 10,000 units per month. Division X sells 7,000 units each month to outside customers, earning a contribution margin of $18 per unit. Division Y would like to buy 4,500 units each month from Division X. How much must Division X add to the per unit transfer price to recover the margin given up from lost external sales?

a. $6.00. b. $12.00. c. $18.00. d. $4.00