The diagram illustrates the income tax system used by a country. What are the average and marginal rates of tax as incomes increase from Y1 towards Y2?
- Aaverage rate of tax: constant; marginal rate of tax: constant
- Baverage rate of tax: constant; marginal rate of tax: decreasing
- Caverage rate of tax: decreasing; marginal rate of tax: constant
- Daverage rate of tax: decreasing; marginal rate of tax: decreasing