Miller Company's contribution format income statement for the most recent month is shown below:

Total Per Unit
Sales (20,000 units) $300,000 $ 15.00
Variable expenses 180,000 9.00
Contribution margin 120,000 $ 6.00
Fixed expenses 70,000
Net operating Income $ 50,000

Which of the following equations explains the difference between the next month’s estimated net operating income ($68,000) and the most recent month’s net operating income ($50,000)?

multiple choice 2
a. 2,000 additional units sold × $9.00 contribution margin per unit = $18,000 increase in net operating income
b. 9,000 additional units sold × $2.00 contribution margin per unit = $18,000 increase in net operating income
c. 3,000 additional units sold × $6.00 contribution margin per unit = $18,000 increase in net operating income
d. 6,000 additional units sold × $3.00 contribution margin per unit = $18,000 increase in net operating income