Truman University is thinking of opening an evening college. In figuring the cost of such a project, a figure is provided for lighting of parking lots. It is pointed out by the university's finance officer that a city ordinance requires that the parking lots be lighted whether there is an evening college in session or not. Lighting expenses in this case are:
a. alternative costs.
b. side effects.
c. sunk costs.
d. opportunity costs.
e. variable costs.