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."
​These changes in the growth rate 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.