According to paragraph 1, which of the following is true of early food production by the end of the Pleistocene? A. In certain areas of western and central Africa, some people were already growing root plants such as yams. B. The cultivation of cereal crops had already been used effectively and widely by people living on the fringes of rain forests. C. Sophisticated forms of "vegeculture" were introduced for the first time in central and western Africa by people from the Sahara's southern borders. D. The cultivation of root and tree crops replaced the cultivation of cereal crops because of decreasing summer rainfall in grassland areas.