The​ short-run aggregate supply curve is horizontal but the​ long-run aggregate supply curve is vertical because:___.
a. nominal GDP is variable in the short run but fixed in the long run.
b. real GDP is assumed to be fixed in the short run but variable in the long run.
c. the price level is assumed to be constant in the short run but completely flexible in the long run.
d. the price level is assumed to be completely flexible in the short run but constant in the long run.