B. The North opposed the extension of slavery into new states.
Explanation:
"Explanation
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In the United States, the North and the South had developed different perspectives on the issue of slavery. As new territories in the West were organized as states, pro-slavery forces in the South wished to make them slave states, while anti-slavery forces in the North wished to make them free of slavery. This was because more representation from the states meant more power in the national government; the Union would be friendlier to slavery if it had more slave states than free states, and it would be more hostile to slavery if it had more free states than slave states.
Many abolitionists in the North wanted to end slavery altogether, but others were willing to contain it by not letting it spread to new states. They believed that this would eventually force slavery to die off; Southern politicians believed the same thing, which is why they wanted slavery to spread."-Study Island
