A columnist in the New York Times notes that the U. labor supply "in the next decade is expected to expand at less than half the pace of the 1960s,...

A columnist in the New York Times notes that the U.S. labor supply​ "in the next decade is expected to expand at less than half the pace of the​ 1960s, 1970s and​ 1980s." 

​Source: Eduardo​ Porter, "The Payoff in Delaying​ Retirement," New York Times​, March​ 5, 2013.

These changes in the growth rates in the U.S. labor supply are the result of

A.

more retirements and fewer families beginning to have two wage earners.

B.

an increase in the immigrant population.

C.

fewer people wanting to work because their incomes are high enough.

D.

a drop in the immigrant population.