21. There are many varieties and grades of paper materials, and whilst it is fairly easy to spot the varieties, it is far more difficult to spot the grades.
纸张材料有许多品种和等级，虽然很容易分辨它们的种类，但要分辨它们的等级就困难得多了。
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22. Borlaug began work at a Rockefeller Foundation-funded project in Mexico to increase wheat production by developing higher-yielding varieties of the crop.
博洛格开始在洛克菲勒基金会资助的墨西哥项目中工作，通过开发高产品种来提高小麦产量。
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23. For example, there are more varieties of fish, or insects, lots of frog spices, and then species that rely on those species start to live near the wetlands too.
例如，有更多种类的鱼，或昆虫，各种青蛙，然后依赖那些物种的物种也开始生活在湿地附近。
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24. The world's population is expected to reach 7,000 million by the year 2000, but already scientists have produced new and better varieties of wheat and rice and animal.
到2000年，世界人口预计将达到70亿，但是科学家已经培育出了新的、更好的小麦、水稻和动物品种。
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25. The plan has good chances of success, since sweet potato is a staple of the region's diet and agriculture, and the varieties currently grown contain little beta-carotene.
该计划成功的机会很大，因为红薯是该地区的主食和主要农产品，而且目前种植的品种几乎不含胡萝卜素。
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26. In 2018, farmers will plant the varieties with the most promise in the Ebro Delta and Europe's other two main rice-growing regions—along the Po in Italy, and France's Rhône.
27. In 2018, farmers will plant the varieties with the most promise in the Ebro Delta and Europe's other two main rice-growing regions—along the Po in Italy, and France's Rhone.
28. Once the idea of planting diffused, Africans began to develop their own crops, such as certain varieties of rice, and they demonstrated a continued receptiveness to new imports.
29. While it is likely that fewer varieties of plant seeds have reached Hawaii externally than internally, more varieties are known to be adapted to external than to internal transport.
30. In the state of Iowa, Diane Ott Whealy wanted to protect historic plant varieties, like the seeds her great-grandfather brought to the U.S. from Germany more than a hundred years ago.
