Mega Bright Window Cleaners' monthly income statement at several levels of activity is as follows: 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. Windows washed gm m m Sales revenue $3,000 $6,000 $9,000 Cost of goods sold LE m m Gross profit m m m Operating expenses Advertising expense 500 500 500 Salaries and wages 700 900 1,100 expense Insurance expense 200 200 200 Postage expense E m LE Total operating expenses 1% m m Operating income ,_$_ {1001 1 000 2 100