Dizzy Amusement Park is open from $8$:$00$ am till midnight every day of the year. Dizzy charges its patrons a daily entrance fee of $$30$ per person which gives them unlimited access to all of the park's $35$ rides.

Dizzy employees a certified operator for each of its $35$ rides. Each operator is paid $$20$ per hour. The cost of the certified operators would best be described as a: fixed cost, step$-$variable cost, true variable cost, or mixed cost.