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Paragraph4: Another important push was the onset of the Little Ice Age, a climatic phenomenon that led to cooler temperatures in the Northern Hemisphere. Although the height of the Little Ice Age was still around the corner, some evidence suggests that temperatures were falling during the thirteenth century. The environmental changes associated with this transition are not fully understood, but people living closest to the San Juan Mountains, to the northeast of Mesa Verde, were affected first. Growing food at these elevations is always difficult because of the short growing season. As the Little Ice Age progressed, farmers probably moved their fields to lower elevations, infringing on the lands of other farmers and pushing people together, thus contributing to the aggregations. Archaeologists identify a corresponding shift in populations toward the south and west toward Mesa Verde and away from higher elevations.
Why does the author state that "Growing food at these elevations is always difficult because of the short growing season"?
○To explain why the higher elevations were always relatively sparsely 稀疏的，稀少的populated
○To suggest that any worsening of conditions would have significant consequences
○To emphasize how resourceful the people growing food at these elevations were
○To argue that farming was not the primary source of food at high elevations