TOEFL Reading: ETS-TOEFL阅读机经 - _I6TFR85H4OV1X74L$

According to paragraph 5, which of the following was the main challenge faced by early agricultural societies that wanted to grow crops on hill slopes? A. Gettingenough irrigation water to the hill slope B. Growing crops without disturbing the natural vegetative cover C. Retaining rainwater and thus preventing excessive runoff D. Identifying crops that do not need a thick rooting zone