Mighty Bright Window Cleaners' monthly income statement at several levels of activity is as follows: Windows washed 1,000 2,000 3,000 Sales revenue $3,000 $6,000 $9,000 Cost of goods sold 1,200 2,400 3,600 Gross profit 1,800 3,600 5,400 Operating expenses Advertising expense 400 400 400 Salaries and wages expense 700 900 1,100 Insurance expense 200 200 200 Postage expense 400 800 1,200 Total operating expenses 1,700 2,300 2,900 Operating income $100 $1,300 $2,500 Required: a. Identify each expense as fixed, variable, or mixed. b.Prepare a contribution margin income statement based on a volume of 5,000 windows.