If Makena's income increases from $40,000 to $50,000 and her tax liability increases from $6,000 to $9,000, which of the following is true? A. Her marginal tax rate is 18% in this range. B. The tax structure must be regressive in the range between $40,000 and $50,000. C. Her marginal tax rate is 30% in this range. D. Her average tax rate was 22.5% when her income was $40,000