(7.6D-2) An economic impact of the agricultural industry resulting from the
close of the frontier -
caused many farmers to declare bankruptcy.
ended cotton farming as a cash crop.
made cotton farming an ideal cash crop.
forced the railroad industry to shut down

made cotton farming an ideal cash crop.

Explanation:

After the close of the frontier, Railroads company took enough ownership of western territory to finish the rails that connected a lot of states.

This make it possible for southern states tho produce large amount of Cotton to distribute their product nationwide. As a result, cotton become an ideal cash crops (plants that generate large profit) . It contributed to a very large portion of southern economy.

location is the answer to this