Which one of the following statements correctly describes the Social Security primary insurance amount $($PIA$)?$

A$)$ PIA is derived from a retiree's average indexed monthly earnings $($AIME$).$

B$)$ PIA is used only to determine a worker's Social Security retirement benefits.

C$)$ PIA will be reduced if a worker begins receiving his Social Security payments prior to full retirement age.

D$)$ The formula for determining one's PIA is weighted so that higher income earners have a higher "income replacement ratio."