A company observed a decrease in the cost per unit. All other things being equal, which of the following is probably true? a. The company is studying a variable cost, and a total volume has increased. b. The company is studying a variable cost, and total volume has decreased. c. the company is studying a fixed cost, and total volume increased d. the company is studying a fixed cost, and total volume decreased e. the company is studying a fixed cost and total volume has remained the same