Spartan Corporation observed that when 25,000 units were sold, a particular cost amounted to $75,000, or $3.00 per unit. When volume increased by 10%, the cost totaled $82,500 (i.e., $3.00 per unit). The cost that Spartan is studying can best be described as a: 

a. Fixed cost

 b. Variable cost 

c. Mixed cost 

d. Sunk cost