The North during this time was more industrialized had more railroads/cities, meaning they had a good economy and did not rely on exports to other countries to pay for their debts.
This being said, it's not A, nor C and D.
The North did not need the South's agricultural assistance, nor natural resources. And, how is the South going to rely on Europe's transportation center?
Thus, B, is your answer.
The South relied heavily on cotton exports to Europe.
