18. Coco's Music Store is deciding whether to continue making guitar strings or to buy the strings from an outside supplier. Coco currently makes her own strings, and the company's cost information is as follows: The special equipment has a salvage value of zero and cannot be used for any other purpose. The supervisor is Coco's grandson Miguel and cannot be fired. What is the total relevant cost of producing batons that Coco should consider in her decision? Assume direct labor is all variable. a. $13.10 b. $15.10 c. $17.60 d. $19.40