Answer:
A. The availability of fertile land was limited.
Explanation:
Before 1840s, poor farmers of the South were the ones that lived from small and poor lands, and did not have slaves. On the other hand, some plantation owners lived from big and rich plantations, owning many slaves. They produced their own food and dedicated some part of the land to produce cotton, sugar and tobacco for export. But eventually, these lands lost their fertility, and the plantation had to be moved to different land in the west.