The correct answer is Westward expansion led to an economic boom in the Midwest, as new cities and markets were connected by rail and canals
Towns started sprawling up and this caused a lot of problems in political balance of the country since those in the south got more slave states and became wealthier and could bother northerners in the senate. The northerner's economy couldn't develop as fast since most of them didn't have slaves so they had to pay a lot of profit to workers.
