Answer:
A. Helped to build the nation into an industrial giant.
Explanation:
Unskilled labor, both native-born, and from abroad (mainly immigrants from Europe), helped provide the needed labor force for the booming industry that developed after the Civil War, and that led the United States to become the most industrialized nation in the world by the start of the twentieth century.
However, the industrialization was geographically uneven: it mostly concentrated in the Northeast and the Great Lakes states, while the South continued to be rural, and stagnated economically.