According to the figure below,the most common source of energy use in the United States in 2000 was ____________. 
Definitions:
Specialization
The concentration of the productive efforts of individuals or firms on a limited number of activities, leading to increased efficiency.
Production
The process of creating goods or services by combining various inputs like labor, materials, and technology.
Hyperinflation
An extremely high rate of inflation, usually defined as an inflation rate in excess of 50 percent per month.
Nationalization
The process by which a government takes control of a private industry, business, or assets and converts it into state ownership and operation.
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