J-Mobile provides cellular phone and Internet services with a plan that provides up to 1,000 minutes of airtime usage for a flat rate of $99.99 per month plus a charge for any minutes used over this amount at the rate of $0.10 per minute in excess of the minimum 1,000 airtime minutes. Which of the following describes the cost of the J-Mobile service?

a. fixed cost, only.

b. variable cost, only.

c. semi-variable cost.

d. semi-fixed cost

why the answer is C?