The Natural Rate of Unemployment in the United States Has

The natural rate of unemployment in the United States has

A) held remarkably constant over the past two decades.
B) fallen slowly over the past 10 years after climbing throughout the 1960s and early 1970s.
C) increased gradually during the 1960s, 1970s, and 1980s, but appears to now lie somewhere between 5 and 6 percent.
D) been so muddled by the large influx of women into the labor market in the 1970s that it is no longer computed.
E) been unaffected by the oil shocks of the 1970s.