Current Attempt in Progress Sheffield Window Cleaners' monthly income statement at several levels of activity is as follows: Windows washed 2,500 4,200 7,000 Sales revenue $ 3,700 $6,216 $ 10,360 Cost of goods sold 1,300 2,184 3,640 Gross profit 2,400 4,032 6,720 Operating expenses Advertising expense 400 400 400 Salaries and wages expense 750 920 1,200 Insurance expense 200 200 200 Postage expense 400 672 1,120 Total operating expense 1,750 2,192 2,920 Operating income 650 $ 1,840 $ 3,800 (a) Identify each expense as fixed, variable, or mixed. Cost of goods sold Advertising expense