The graph below shows the per capita gross domestic product (GDP) in the United States from 1940 to 1980. Per capita GDP measures each person's share of the economy:

Which factor explains the trend shown on this graph?

growth of the U.S. population resulting from the baby boom
growth of U.S. productivity resulting from industrial expansion
growth of the federal budget resulting from increased defense spending
growth of imports resulting from increased consumer borrowing and
spending