13. The process of evaluating the credit policy to determine whether a shift in the customers payment patterns has occurred is called a. receivables monitoring b. payables management c. concentration banking d. a lockbox system e. relaxed management 14. Which of the following mathematical expressions is true if the net operating income of a firm is equal to zero? a. Sales -Gross profit margin b. Sales = Total variable costs + Total fixed costs c. Total operating costs Total variable costs +Total contribution margin costs d. Total operating costs Contribution margin e. Contribution margin Earnings before interest and tax