HELP!!! How did railroads help define the era and benefit the nation?

A.They united the country by making tariffs profitable for each region: North, South, and West.

B.They helped create one integrated economy, moving raw materials from farms in the North to factories in the South.

C.They were used to ship goods and crops across the nation and helped achieve unprecedented growth.

D.They strengthened democracy in the United States by making election results known in a timelier manner.