Which of the following pieces of evidence would by itself allow a judgment about the regressivity or progressivity of a tax?

a. On average, high income families have higher tax bills than do low income families.

b. On the whole, the richest five percent of the population pay thirty percent of the total tax collected.

c. In general, per capita tax payments by rich families are higher than per capita tax payments by poor families.

d. Upper income families have a lower share of the nations after-tax income than they do of the nations before-tax income.