Columbia Inc. has provided the following for three months of income statements: January February March Sales in Units 3,000 3,500 4,000 Sales 900,000 1,050,000 1,200,000 Less: cost of goods sold 540,000 630,000 720,000 Gross margin 360,000 420,000 480,000 Operating expenses: Advertising 25,000 25,000 25,000 Salaries & Commissions 175,000 197,500 220,000 Depreciation expense 25,000 25,000 25,000 Utilities expense 75,000 87,000 99,000 Total operating expenses 300,000 334,500 369,000 Net Income 60,000 85,500 111,000 Classify each of the company's expenses as variable, fixed, or mixed.