Which of the following is not correct?

a. With a growth rate of about 2 percent per year, average income per person doubles about every 60 years.

b. In some countries real income per person has changed very little over many years.

c. Across countries there are large differences in the average income per person. These differences are reflected in large differences in the quality of life.

d. The ranking of countries by average income changes substantially over time.