Which of the following most contributed to American economic growth and prosperity in the 1920s?
A. Adoption of the Bessemer process by steel producers
B. An increase in ownership of automobiles
C. The invention of the telephone for long-distance communications
D. The extension of railroad lines across the country

The extension of railroad lines across the country

Explanation:

In economics, capital and innovation flows more depending on how much infrastructure connects the area. Its the reason why California in 1860 had no where near the manufacturing and industry of New England, because it lacked infrastructure to carry supplies and workers.

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