Over the past 40 years, the richest 20 percent of the U.S. population has earned

a. Less income than the poorest fifth of the U.S. population. 

b. A declining share of total money income in the U.S. 

c. A constant share of total money income in the U.S. 

d. An increasing share of total money income in the U.S. 

e. None of the above