单项选择题

TEXT A
Slavery Divides the Nation
Conflicting interests in north and south became increasingly apparent. Resenting the large profits obtained by northern businessmen from marketing the cotton crop, southerners attributed the backwardness of their section to northern expansion. Northerners, on the other hand, declared that slavery--the "peculiar institution" the south felt to be essential to its economy-- was wholly responsible for the region’s relative backwardness.
As far back as 1830s, sectional lines had been steadily hardening on the slavery question. In the north, abolitionist feeling grew more and more powerful, encouraged by a free-soil movement vigorously opposed to the extension of slavery into the regions not yet organized as states. To southerners of 1850, slavery was a condition for which they were no more responsible than for their English speech or their representative institutions. In some coastal areas, slavery by 1850 was well over 200 years old, an integral part of the basic economy of the region. In 15 southern and border states, the black population was approximately half as large as the white, while in the north it was an insignificant fraction.
From the middle 1840s, the slavery issue became more important than all else in American politics. The south, from the Atlantic to the Mississippi River and beyond, was a relatively compact political unit agreeing on all fundamental policies affecting cotton culture, using only primitive implements, was singularly adapted to the employment of slaves. It provided work nine months of the year and permitted the use of women and children as well as men.
Why did southerners want to keep slavery