Concord Window Cleaners' monthly income statement at several levels of activity is as follows Windows washed 2,000 4,200 8,200 $ 3,900 $ 8,190 $15,990 Sales revenue Cost of goods sold Gross profit Operating expenses 1,400 2,9405,740 2,500 5,250 10,250 400 9201,320 200 9451,845 1,7502,465 3,765 $750 $ 2,785 $ 6,485 400 700 200 450 Advertising expense 400 Salaries and wages expense Insurance expense 200 Postage expense Total operating expense Operating income (a) Identify each expense as fixed, variable, or mixed Cost of goods sold Advertising Salaries and Wages Insurance Postage variable fixed mixed fixed variable