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  3. With the world’ s population estimated to grow from six to nine billion by 2050, researchers, businesses and governments are alr

With the world’ s population estimated to grow from six to nine billion by 2050, researchers, businesses and governments are alr

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问题    With the world’ s population estimated to grow from six to nine billion by 2050, researchers, businesses and governments are already dealing with the impact this increase will have on everything from food and water to infrastructure (基础设施) and jobs. Underlying all this 【C1】______ will be the demand for energy, which is expected to double over the next 40 years.
   Finding the resources to meet this demand in a 【C2】______, sustainable way is the cornerstone (基石) of our nation’ s energy security, and will be one of the major【C3】______of the 21st century. Alternative forms of energy—bio-fuels, wind and solar, to name a few— are 【C4】______ being funded and developed, and will play a growing【C5】______in the world’ s energy supply. But experts say that, even when【C6】______, alternative energy sources will likely meet only about 30% of the world’ s energy needs by 2050.
   For example, even with【C7】______investments, such as the $93 million for wind energy development 【C8】______ in the American Recovery and Reinvestment Act, important alternative energy sources such as wind and bio-fuels 【C9】______ only about 1% of the market today.
   Energy and sustainability experts say the answer to our future energy needs will likely come from a lot of【C10】______—both traditional and alternative.
   A) stable B) solutions C) significant D) role
   E) progress F) marvelous G) included H) growth
   I) exactly J) consist K) comprise L) competitions
   M) combined N) challenges O) certainly
【C4】
选项
答案O
解析 本句话不缺少成分，意义也完整，且在实义动词之前，可推测这里应该是副词，certainly更符合题意。
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大学英语四级

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