Intuit Inc. develops and sells software products for the personal finance market, including popular titles such as Quicken and TurboTax. Classify each of the following costs and expenses for this company as either variable or fixed to the number of units produced and sold: Property taxes on general offices President's salary Wages of telephone order assistants Salaries of human resources personnel Salaries of software developers Packaging costs CDs Sales commissions Straight-line depreciation of computer equipment Users' guides Shipping expenses