A. stable     B. solutions    C. considerable    D. role             E. included F. task      G. compose     H. impact         I. challenges         J. certainly

With the world's population growing from six to nine billion by 2050, researchers, businesses and governments are already dealing with the ______ this increase will have on everything from food and water to infrastructure (基础设施) and jobs. The demand for energy will keep growing, and is expected to double over the next 40 years.
Finding the resources to meet this demand in a ______, sustainable way is the basis of our nation's energy security, and will be one of the major ______of the 21st century. Alternative forms of energy- bio-fuels, wind and solar are _______ being funded and developed, and will play a growing______ in the world's energy supply. But experts say that even when combined, alternative energy sources will likely meet only about 30% of the world's energy needs by 2050.
For example, even with ______ investments, such as the $93 million for wind energy development ______ in the American Recovery and Reinvestment Act, important alternative energy sources such as wind and bio-fuels _____ 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 ______ both traditional and alternative.