Correct. The answer is b. To support his assertion that the 1846 dispute over slavery in the Mexican Cession broke down along sectional lines, the author demonstrates that Northerners, regardless of their party affiliation, supported banning slavery in the territory, but Southerners of all parties opposed Wilmot’s proposal because it banned slavery in the territories.
Incorrect. The answer is b. To support his assertion that the 1846 dispute over slavery in the Mexican Cession broke down along sectional lines, the author demonstrates that Northerners, regardless of their party affiliation, supported banning slavery in the territory, but Southerners of all parties opposed Wilmot’s proposal because it banned slavery in the territories.