Five months ago Wilson opened up a health club. Which of the following is an implicit cost related to the health club?

a$.$ Wilson previously worked as an accountant, earning $$3,000$ a month.

b$.$ Wilson usually eats four hamburgers a day, priced at $$3$ each.

c$.$ Wilson paid $$120$ for an outside laundry service to clean the towels used at the club.

d$.$ Wilson paid $$100$ for the pest control exterminator to spray the health club.