The following website, maintained by the Federal Reserve Bank of St. Louis, contains information on the labor force participation rate measured monthly from 1950 to the present: https://fred.stlouisfed.org/series/CIVPART. One of the most striking features of this graph, is the steady and large increase of the labor force participation rate from about 1960 to 2000.

A. Alluding to the mathematical definition of the labor force participation rate, explain what must be happening to the size of the labor force relative to the size of the working age population from 1960 to 2000 to explain the increase observed.