TOEFL Reading: ETS-TOEFL阅读机经 - KMSB6SB4Y135V7K2L$

According to paragraph 2, before 1970, scholars believed that Chacoan society collapsed because A. Chaco never had the forests that were needed for the development of a stable agricultural economy. B. farmers used up the natural resources in Chaco that had originally allowed the society to succeed. C. Chaco suffered a long-term drought that prevented farmers from growing enough food. D. laborers left Chaco to find other work after they finished building the great houses there.