Which of the following statements is true? I. Under variable costing, fixed manufacturing overhead is treated as a product cost. II. Under variable costing, all variable production costs are treated as product costs. III. Under variable costing, an increase in fixed manufacturing overhead will affect the unit product cost. Which of the following statements is true? I. Under variable costing, fixed manufacturing overhead is treated as a product cost. II. Under variable costing, all variable production costs are treated as product costs. III. Under variable costing, an increase in fixed manufacturing overhead will affect the unit product cost. Only statement I is true. All statements are true. Only statement II is true. None of the statements are true