At the Snow Speciality Derma Clinic, each customer is required to pay a minimum cost of $40 in addition to the regular charges. This cost includes $25 for the person providing the services and $15 for the equipment used. This sum of $40 is referred to as the:
a) marginal cost of a service.
b) inventory cost of a service.
c) social cost of a service.
d) sunk cost of a service.
e) opportunity cost of a service.
Answer: B