Which of the following findings would most strongly support the assertion made by the archaeologists mentioned in the first paragraph? A. A population remained in a certain region at least a century after erosion wore away much of the topsoil that sustained grass for their grazing animals. B. The range of a certain group's agricultural activity increased over a century of gradual decrease in annual rainfall. C. As winters grew increasingly mild in a certain region, the nomadic residents of the region continued to move between their summer and winter encampments. D. An agricultural population began to trade for supplies of a grain instead of producing the grain in its own fields as it had in the past. E. A half century of drought and falling groundwater levels caused a certain population to abandon their settlements along a riverbank.