Since the end of World War II, the U.S. has almost always had rising prices and an upward trend in real GDP. To explain this a. both aggregate demand and long-run aggregate supply must be shifting right and aggregate demand must shift farther. b. it is only necessary that aggregate demand shifts right over time. c. it is only necessary that long-run aggregate supply shifts right over time. d. None of the above cases would produce rising prices and growing real GDP over time.