Final answer:
The spread of industrialization was the greatest influence on the U.S. Civil War and Reconstruction in the 20th century.
Step-by-step explanation:
The greatest influence in the 20th century on the U.S. Civil War and Reconstruction was D) The spread of industrialization. After the Civil War, the United States underwent massive industrialization, with major industries developing and growing in the eastern half of the country. This industrial growth fueled the expansion of railroads, steel manufacturing, and other industries, transforming the country from a rural to an urban society. The industrial revolution impacted various aspects of American life, including the economy, politics, and social dynamics.