Under absorption costing, all manufacturing costs are treated as product costs. This includes direct materials, direct labor, variable manufacturing overhead, and fixed manufacturing overhead. These costs are assigned to inventory and expensed as cost of goods sold when the goods are sold. Selling expenses, however, are not manufacturing costs. They are period costs and are expensed in the period incurred. Option B is incorrect because fixed manufacturing overhead is included in product cost under absorption costing. Options C and D are also product costs because they relate directly to manufacturing. Internal auditors reviewing costing systems should verify that product and period costs are classified properly, because misclassification affects inventory valuation and operating income. Therefore, Option A is correct.