$--$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 $$99\mathrm{.}99$ per month plus a charge for any minutes used over this amount at the rate of $$0\mathrm{.}10$ per minute in excess of the minimum $1,000$ airtime minutes. Which of the following describes the cost of the J$-$Mobile service?

fixed cost, only.

mixed cost.

variable cost, only.

semi$-$fixed cost.