The "natural" rate of unemployment is the unemployment rate toward which the economy gravitates in the
a. short run, and the natural rate is not necessarily the socially optimal rate of unemployment.
b. long run, and the natural rate is the socially optimal rate of unemployment.
c. long run, and the natural rate is not necessarily the socially optimal rate of unemployment.
d. short run, and the natural rate is the socially optimal rate of unemployment.