Since Anytime Pizza is open 24 hours a day, its pizza oven is constantly on and is, therefore, always using natural gas. However, when there is no pizza in the oven, the oven automatically lowers its flame and reduces its natural gas usage by 70%. The cost of natural gas would best be described as a:

Select one:

a. true variable cost.

b. step-variable cost.

c. fixed cost

d. mixed cost.