Birch company manufactures a part for its production cycle. the costs per unit for 5,000 units of this part are as follows: Direct materials $3 Direct Labor $5 Variable factory overhead $4 Fixed factory overhead $4 Total costs $16 The fixed factory overhead costs are unavoidable. Spalding corporation has offered to sell 5,000 units of the same part to Birch company for $15 a unit. Assuming no other use for the facilities, Birch company should______ a. buy from scalding corp to save $1 per unit b. make the part to save $1 per unit c. buy from scalding corp to save $3 per unit d. make the part to save $3 per unit