As indicated in the chapter, the CBO forecast that real GDP would grow at an average annual rate of 2.1 percent from 2019 to 2029. The Trump administration pledged to raise the growth rate to 3 percent, although some policymakers and economists were skeptical that this goal could be achieved. Yet from 1960 to 1969, real GDP grew at an average annual rate of 4.5 percent. Briefly discuss the factors that make growth rates that high more difficult to achieve in the current U.S. economy than in the economy of the 1960s.