4. If the Phillips curve is steeper at higher rates of inflation, it suggests that

a. once the economy has relatively low unemployment rates, further reductions in the unemployment rate can occur only by accepting larger increases in the inflation rate.

b. once the economy has relatively low unemployment rates, further reductions in the unemployment rate can occur only by reducing the rate of inflation.

c. only reducing the rate of inflation can reduce the high unemployment rate.

d. When the economy is experiencing low inflation rates, the unemployment rate can be reduced only by accepting lower rates of inflation.