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. Underling 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 2049.
    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
【C3】

选项

答案N

解析 根据空格前的one of the major可推断此处需填入名词的复数形式。上文提到能源需求不断增加,此空所在段的段末谈及可替换能源或许只能满足世界能源需求的30%左右,由此可见,要找到充足的能源是一种“挑战”,challenges符合文意。competitions多指人或机构为了某种利益而竞争,此处没有“抢夺能源”这种语境,所以排除。
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