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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. 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
【C6】
选项
答案M
解析 分析句子结构可知，when引导的时间状语从句省略了主语和be动词，结合句意可知，空格处的单词与alternative energy sources是被动关系，故此处应填动词的过去分词形式。由句子大意可推断，此处所填的词有“合在一起，结合”的意思，故combined(使结合，联合)符合句意。
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