How did both mountains and high plateaus influence the rise and growth of empires in mainland southeast asia? select all that apply. a. mountains and high plateaus protected the regional empires that included the four main river valleys of southeast asia. b. mountains and high plateaus made it difficult for the rest of asia to trade with and conquer mainland southeast asia. c. mountains and high plateaus encouraged cultural diffusion in the empires that arose in mainland southeast asia. d. mountains and high plateaus prevented civilizations from developing along the four main southeast asian river valleys.