A central lesson of the Solow model is:______.a. capital accumulation cannot serve as the engine of long-run per capita economic economic growth. b. capital accumulation is the only engine of the long run per capita economic growth. c. consumption is the only engine of short-run per capita economic growth.d. saving rates serve as the engine of long-run per capita economic growth. d. the initial capital stock plays a large part in long run economic growth.