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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 alre

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

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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】______ and 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 I) exactly
B) solutions J) consist
C) significant K) comprise
D) role L) competitions
E) progress M) combined
F) marvelous N) challenges
G) included O) certainly H) growth
【C5】
选项
答案D
解析 分析该句的句子结构可以推断出空格处应为名词单数。备选项中的role经常与play搭配，构成固定词组play a role in，意为“在……发挥作用”，此处role的意思是“作用”。
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