All of the choices in some ways gave rise to the early civilizations. Mountains helped India to be protected from China, the ocean helped for food, the Sahara desert protected lower Africa from invasion for many centuries. Rivers also helped since they were a major trade routes and the most fertile soil. I think that the Rivers were the most important geographic feature. So the correct answer is C.
From the brightness of the light of course. If the brightness is increasing it means the the light source is approaching and if the brightness is decreasing it means the light source is leaving.
