Which of the Statements Below Is Incorrect Regarding Commission Rates

Which of the statements below is incorrect regarding commission rates?

A) progressive rates, which increase the percent of commission awarded as sales volume increases, are best when profit margins climb sharply after the break-even point is reached
B) regressive commission rates are usually utilized when there is a high probability of windfall sales or propensity to overload customer inventories
C) fixed commission rates are the easiest to calculate
D) none of the above are incorrect