Identify the nature of the following costs (i.e., fixed, variable, stepped, mixed, etc.) based on their behavior, and define how they would be influenced by changes in volume of production on both an in total and per unit basis. Units Sold 20 40 60 80 100 a. Rental cost per unit of merchandise $36 $18 $12 $9 $7.20 sold b. Total phone expense $80 $100 $120 $140 $160 C. Total cost of goods sold $1,800 $3,600 $5,400 $7,200 $9,000