Answer:
I think A
Step-by-step explanation:
Why?
Well we can do some elimination first. We know that the North had more factories. The North also had more miles or railroad because they had slaves underground but don't get that mixed up with the underground railroad. The higher population might be it but if I recall correctly they South most fought on their own land since the North was super cold.
But I could be wrong. If I could chose more then one then it would be A,D.
Answer:A