TOEFL Listening: TPO-TOEFL阅读TPO - M3X7R13W1531K714N$

Which of the following is NOT mentioned in paragraphs 5 and 6 as one of the strategies the Chinese applied in agriculture? A. The growing of two crops on the same field during the same year B. The improvement of systems to supply crops with water C. The application of increasing amounts of fertilizer to the land D. The reduction in the amount of human labor per unit of land