With a Tax of $4,000 on $20,000 of Income and $6,000

With a tax of $4,000 on $20,000 of income and $6,000 on $30,000 of income, the average tax rate is

A) constant at 10 percent.
B) lower than the marginal tax rate.
C) equal to the marginal tax rate.
D) higher than the marginal tax rate, which is equal to zero.