The table shows the natural gas price index of natural gas delivered to residential consumers in the United States. Consumers are charged per thousand cubic feet.

During which years did the price of natural gas decrease? Separate the years in your answer with commas.

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National Gas Price Index Year CPI Year CPI Year CPI 1966 1981 70.8 1996 104.6 1967 17.2 1982 85.3 1997 114.5 1968 17.2 1983 100.0 1998 112.5 1969 17.3 1984 101.0 1999 110.4 1970 18.0 1985 101.0 2000 128.1 1971 19.0 1986 96.2 2001 158.9 1972 20.0 1987 91.4 2002 130.2 1973 21.3 1988 90.3 2003 158.9 1974 23.6 1989 93.1 2004 177.4 1975 28.2 1990 95.7 2005 209.6 1976 327 1991 96.0 2006 226.6 1977 38.8 1992 97.2 2007 215.8 1978 42.2 1993 101.7 2008 229.2 1979 49.2 1994 105.8 2009 200.3 1980 60.7 1995 100.0 2010 National Gas Price Index Year CPI Year CPI Year CPI 1966 1981 70.8 1996 104.6 1967 17.2 1982 85.3 1997 114.5 1968 17.2 1983 100.0 1998 112.5 1969 17.3 1984 101.0 1999 110.4 1970 18.0 1985 101.0 2000 128.1 1971 19.0 1986 96.2 2001 158.9 1972 20.0 1987 91.4 2002 130.2 1973 21.3 1988 90.3 2003 158.9 1974 23.6 1989 93.1 2004 177.4 1975 28.2 1990 95.7 2005 209.6 1976 327 1991 96.0 2006 226.6 1977 38.8 1992 97.2 2007 215.8 1978 42.2 1993 101.7 2008 229.2 1979 49.2 1994 105.8 2009 200.3 1980 60.7 1995 100.0 2010