Over the last century, hours of work per lifetime have declined about 50 percent while real earnings have increased by a factor of 8.8. Assuming that the main change was an increase in the marginal-productivity of-labor schedule, draw supply-and-demand diagrams for labor in 1900 and 2000 that will explain this trend. In your diagrams, put the number of hours worked per lifetime on the horizontal axis and the real wage rate on the vertical axis. What key factor about the supply of labor must you invoke to explain this historical trend?