Directions: Read the following text from which 10 words have been removed. Choose from the words A-O the most suitable one to fill each numbered gap in the text (46-55). There are FIVE extra words that you do not need to use. Mark your answers on your ANSWER SHEET 1.

　　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 46 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 47, sustainable way is the cornerstone(基石)of our nation's energy security, and will be one of the major 48 of the 21st century. Alternative forms of energybiofuels, wind and solar, to name a few--are 49 being funded and developed, and will play a growing 50 in the world's energy supply. But experts say that even when 51 , alternative energy sources will likely meet only about 30% of the world' s energy needs by 2049.

　　For example, even with 52 investments, such as the $ 93 million for wind energy development 53 in the American Recovery and Reinvestment Act, important alternative energy sources such as wind and bio fuels 54 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 55--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