Wishy Washers Window Cleaners' monthly income statement at several levels of activity is as follows: Windows washed 1,000 2,000 Sales revenue $3,000 $6,000 3,000 $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 equired Identify each expense as fixed, variable, or mixed. Cost of Goods Sold Advertising expense Salaries and wages expense Insurance expense Postage expense Prepare a contribution margin income statement using proper form based on a volume of 2,500 windows.