Which of the following examples illustrates a proportional income tax?

a. I earn $5,000 and pay $500 in taxes; you earn $10,000 and pay $1,000 in taxes.
b. I earn $5,000 and pay $500 in taxes; you earn $10,000 and pay $500 in taxes.
c. I earn $5,000 and pay $500 in taxes; you earn $10,000 and pay $800 in taxes.
d. I earn $5,000 and pay $500 in taxes; you earn $10,000 and pay $1,200 in taxes.

A