answer the next question on the basis of the following four tax schedules for the given base of taxable income. income tax a tax b tax c tax d $10,000 $650 $1,000 $500 $1,000 20,000 850 2,000 1,000 3,000 30,000 950 4,000 1,500 6,000 40,000 1,050 6,000 2,000 9,500 which of the above tax schedules is a proportional tax schedule throughout?