Why Did Northerners Oppose President Polk's Proposal to Extend to the Pacific

Why did Northerners oppose President Polk's proposal to extend to the Pacific coast the 36° 30 line that separated free states from slave states?

A) Most of the lands gained from Mexico were below this line and likely to enter as slave states.
B) The Mexican territory was not part of the original Louisiana Purchase so this line did not apply to it.
C) Mexico threatened a second war if slavery was permitted in its former territory.
D) Former Mexican territory north of this line would still be slave states.