Since 1970 in the United States,the Distribution of Income Has

Since 1970 in the United States,the distribution of income has changed and the group that benefited the most has been the

A) better educated.
B) less educated.
C) low-skilled workers.
D) middle class, that is the group in the middle 20 percent of the income distribution.
E) None of the above answers is correct because all groups benefited equally.