Vigorous political debate accompanied territorial expansion of the united states before the civil war because

Vigorous political debate accompanied territorial expansion of the united states before the civil war because any new states admitted to the Union might upset the sectional balance

The Westward expansion of the United States led to the expansion of the slavery into the Western territories which created a great deal of controversy, thus, causing a huge gap between the North and the South which acted as a factor in causing the Civil War later on.

The answer would be a