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Since the discovery of the Mayan ruins in the 1830s, researchers have tried to explain the collapse of Mayan civilizations. A. The Mayan attempt to develop intensive agricultural methods to support large populations in relatively small areas probably was unsuccessful and could have caused the Mayan collapse. B. The discovery of intact pyramids submerged in vegetation among the Mayan ruins led researchers to believe that Mayan cities were simply overgrown rather than catastrophically destroyed. C. Warfare and rivalry between Mayan states may have caused food shortages that contributed to the eventual collapse of Mayan civilization. D. Early theories that the Mayan collapse was a sudden, catastrophic event were followed by views that treated the collapse as a gradual process. E. The continuing warfare and social unrest that started in the North and spread to the South provided researchers with evidence that the Mayan collapse took hundreds of years to occur. F. Drought between A.D. 800 and A.D. 1000 likely caused the Mayan systems of intensive irrigated agriculture to fall, which could have brought about a rapid collapse of the Mayan states.