TOEFL Reading: ETS-TOEFL阅读机经 - 43HN1_TC95SH5729L$

According to paragraph 3, recent studies claim which of the following about the Mayan collapse? A. It was caused primarily by frequent wars between rival city-states. B. It was caused by a single, sudden event. C. It was preceded by social unrest in northern city-states. D. It began in southern city-states and spread to others.