RELEVANT COSTING (make or buy)

THE QUESTION IS FROM KAPLAN KIT:
QS 67

P is considering whether to continue making a component or to buy it from an outside supplier. It uses 12,000 of the components each year.
The internal manufacturing cost comprises:

$/unit Direct materials 3.00
Direct labor 4.00
Variable overhead 1.00
Specific fixed cost 2.50
Other fixed costs 2.00
12.50

If the direct labor were not used to manufacture the component, it would be used to increase the production of another item for which there is unlimited demand. This other item has a contribution of $10.00 per unit but requires $8.00 of labor per unit.
What is the maximum price per component, at which buying is preferable to internal manufacture?

A $8.00
B $10.50
C $12.50
D $15.50

ANSWER: D

Direct material 3.00
Direct labor (W1) 9.00
Variable overhead 1.00
Specific fixed cost 2.50
15.50

(W1) Relevant cost = Contribution Forgone + Direct labour = $10/2 + $4 = $9

DOUBT:

WHY ARE THEY DIVIDING 10 BY 2? I CALCULATED RELEVANT COST AS: 10+4=14