Plaza Corporation observed that when 25,000 units were sold, a particular cost amounted to $70,000, or $2.80 per unit. When volume increased by 15%, the cost totaled $80,500 (i.e., $2.80 per unit). The cost that Plaza is studying can best be described as a:

variable cost.

fixed cost.

semi variable cost.

discretionary fixed cost.

step-fixed cost.