Use the following information to answer questions 9-10 Comparative income statements for Tudor Retailing Company for the last two months are presented below: September October Sales in units 5,000 7,000 Sales revenue $100,000 $140,000 Cost of goods sold 40,000 56,000 Gross margin 60,000 84,000 Selling and administrative expenses: Shipping expense 7,500 10,500 Clerical expense 10,000 12,000 Maintenance expense 17,000 17,000 Total selling and administrative 34.500 39.500 expense Net operating income $25.500 $44.500 Which of the following classifications best describes the behavior of shipping expense?