TABLE B.16 U.S. Gross Domestic Product Year GDP, Current Dollars (billions) GDP, Real (1996) Dollars (billions) 1976 1,823.9 4,311.7 1977 2,031.4 4,511.8 1978 2,295.9 4,760.6 1979 2,566.4 4,912.1 1980 2,795.6 4,900.9 1981 3,131.3 5,021.0 1982 3,259.2 3,534.9 4,919.3 1983 5,132.3 1984 3,932.7 5,505.2 1985 4,213.0 5,717.1 1986 4,452.9 5,912.4 1987 4,742.5 6,113.3 1988 5,108.3 6,368.4 1989 5,489.1 6,591.8 1990 5,803.2 6,707.9 1991 5,986.2 6,676.4 1992 6,318.9 6,880.0 7,062.6 1993 6,642.3 1994 7,054.3 7,347.7 1995 7,400.5 7,813.2 7,543.8 1996 7,813.2 1997 8,318.4 8,159.5 1998 8,781.5 8,508.9 1999 9,274.3 8,859.0 2000 9,824.6 10,082.2 9,191.4 2001 9,214.5 2002 10,446.2 9,439.9 Table B.16 presents data on the U.S. Gross Domestic Product (GDP). Plot the GDP data and calculate the sample autocorrelation function. Is there an indication of nonstationary behavior in the time series? Now plot the first difference of the GDP time series and compute the sample autocorrelation function of the first differences. What impact has differencing had on the time series?