TOEFL Reading: ETS-TOEFL阅读机经 - U5AI1DV_16CY017XL$

People trying to grow crops in the dry southwestern area of North America experimented with various strategies for almost a thousand years. A. Experiments in agriculture succeeded in those areas that experienced climate changes that increased the amount of rainfall gradually over many centuries. B. In spite of usually dry conditions, water runoff from rainstorms was used by the Hohokam to flood fields and deepen channels in irrigation systems. C. Shifting sites or planting in areas with good sources of water was successful only in areas with low population densities. D. Farming at higher elevations meant risking failure from cold weather, while extensive irrigation at the lower elevations risked destructive floods from sudden rainstorms. E. Farming in dry climate phases was successful only at higher elevations. F. Sharing food produced at different sites had limited success, while self-sufficient and ecologically sound farming continued to succeed.