TOEFL Reading: ETS-TOEFL阅读机经 - 8JN2K83PPN8677P5L$

According to paragraph 5, why did the raised fields in many areas become unusable? A. The marshy depressions around the fields flooded in the rainy season. B. Intensive cultivation of the fields drained the soil of nutrients. C. The area where the fields were located experienced a drop in water levels. D. Unstable design caused the failure of the drainage ditches.