In what way did the nation's growing transportation system help promote economic growth?
A:It made it easier for Native Americans and freed African Americans to obtain the same goods as white people, boosting demand.
B:Because it added costs for the delivery of goods, it made more profits for the manufacturers.
C:Railroad rebates and pools brought down prices for small farmers, who could then afford to modernize their equipment.
D:It opened up national markets for selling and obtaining resources and helped lower costs of production.