A "Three-Legged Stool" Refers to What Issue Discussed in the Text

A "three-legged stool" refers to what issue discussed in the text?

A) Poverty rate differences between children, adults, and the elderly.
B) The idea in the United States that elders receive income from three primary sources: Social Security, private pensions, and personal assets and savings.
C) Cumulative advantages and disadvantages associated with sex, race/ethnicity, and social class.
D) Adult children caring for their aging parents and their own children.
E) The failures of Medicare as the health policy program for the elderly.