Kathy Received a Commission of $12,000 from Her Employer and She

Kathy received a commission of $12,000 from her employer and she also earns semimonthly wages of $2,500.Which of the following federal withholding methods is incorrect?

A) If taxes have already been withheld from Kathy's wages separately,the commission is added to the wages and federal taxes are withheld as if the total were a single payment.Subtract the tax already withheld and withhold the remaining tax from the supplemental wages.
B) If taxes have already been withheld from Kathy's wages,the commission is taxed at a flat rate of 28%.
C) If taxes have not already been withheld from Kathy's regular wage,the commission is added to the wages and withheld as if the total were a single payment.
D) If taxes have already been withheld from Kathy's wages separately,the commission is subject to backup withholding of 25%.