Why Is the Long-Run Aggregate Supply Curve a Vertical Line

Multiple Choice

Why is the long-run aggregate supply curve a vertical line?

A) At that level of real GDP, the unemployment rate is 0 percent.
B) At that level of real GDP, the inflation rate is 0 percent.
C) At that level of real GDP, the production costs are at their lowest level.
D) At that level of real GDP, production costs have fully adjusted to price changes.