Think of it like this.
Your on your own for the first time and you rely on your job as your source of money. With your job you earn a lot of money which you are able to use to buy what is need to keep you alive and going.
Now, your the US, and you job represents the slaves and the south. The new independent couldn't count any anyone but themselves for money and resources. The south (Which was run mostly off slaves) was the key to the new independent country's economy. So would you quit your job (give up on slavery) so you can benefit another worker (help the slaves) and then end up going broke with nothing but land with no money? Didn't think so. * I do agree that slavery is bad, what I wrote was putting my self in the shoes of the new independent country, I didn't mean to offend any one in any way and if I did, I do apologize*.
The answer would be C! Hope I helped. :)