Single Change in the Last Several Decades Has Had the Largest

single change in the last several decades has had the largest positive effect on the average income of current elderly persons in the United States?

A) More women have worked, and so earn retirement income.
B) Men are living longer, so there are more couples earning the higher male retirement incomes.
C) Individual Retirement Accounts (IRAs) were created.
D) Social Security benefits have significantly increased.