4. The main difference between the classical model of the price level and Keynesian economics is that

a. the classical model assumes a vertical short-run aggregate supply curve.

b. Keynesian economics assumes a vertical short-run aggregate supply curve.

c. the classical model assumes an upward sloping longrun aggregate supply curve.

d. Keynesian economics assumes a vertical long-run aggregate supply curve.

e. the classical model assumes aggregate demand can not change in the long run.