The following is Atari Corporation's contribution format income statement for last month: Sales: $1,200,000 Variable Expenses: $900,000 Contribution Margin: S ?????? Fixed Expenses: $150,000 Net Operating Income: $ ?????? The company has no beginning or ending inventories. The company produced and sold 20,000 units last month. If sales increased by 300 units, by how much should net operating income increase? a $18,000 O b. $4,500 c. $300 O d. $6,000