The answer is A. Northerners were excited about adding new states because it could shift the balance of power in Congress.
By adding new states to the Union, the number of representatives each state had in Congress would change, potentially shifting the balance of power between the North and South. Northerners viewed this as an opportunity to increase their influence in Congress and advance their political agenda. This was particularly important in the years leading up to the Civil War, when tensions between the North and South were high and the issue of slavery was a major point of contention.