Answer (C) is correct. In a variable costing system, only the variable costs are recorded as product costs. All fixed costs are expensed in the period incurred. Because changes in the relationship between production levels and sales levels do not cause changes in the amount of fixed manufacturing cost expensed, profits more directly follow the trends in sales, especially when the UCM (selling price per unit – variable costs per unit) is constant. Unit sales times the UCM equals the total CM, and operating income (a pretax amount) equals the CM minus fixed costs of operations. If the UCM is constant and fixed costs are stable, the change in operating income will approximate the change in the CM (unit sales × UCM).