C. Decrease by $24,000. Start by multiplying the selling price by the number of units sold, or $23/unit by 12,000 units, to get $276,000, or revenue. Then find the total variable expense costs by summing each variable cost per unit, for $8.10/unit. Multiply this by 12,000 units then subtract the variable cost and the fixed cost of $100,000 from the revenue, to get $78,800, or gross income.
Next, find the new gross income by adding $2.00 to the variable cost per unit, to get $10.10/unit and multiplying by 12,000 units and subtracting this variable cost and the fixed cost from the revenue to get $54,800.
To find the contribution margin, subtract the original gross income from the new gross income to get a decrease of $24,000.