TOEFL Reading: ETS-TOEFL阅读机经 - M84KY48KBC365OH4L$

In paragraph 3, why does the author discuss crops grown in Asia, India, the African savanna, and the New World A. To show how widely the understanding of the benefits of combining legumes and cereals was applied B. To suggest that it was most effective for the same crops to be grown year after year in many parts of the world C. To emphasize that proteins that come from plants were recognized as valuable in many parts of the world D. To demonstrate that a wide variety of very different cereals and legumes could be grown together