Which Is an Example of How Farming Changed in America

Which is an example of how farming changed in America between 1800 and 1840?

A) Cyrus McCormick's reaper made possible a boom in cotton production.
B) Farmers in the Old Northwest increasingly relied on slave labor.
C) Farmers in the West were able to ship crops and livestock to eastern markets.
D) Corn production shifted to the "hill farms" of the New England and Mid-Atlantic states.
E) America became one of the world's leading exporters of wheat.