Answer:
[tex]{\boxed {\sf B. \ People \ thought \ differently \ about \ slavery \ because \ the \ book \ showed \ how \ horrific \ it \ was}}[/tex]
Explanation:
Uncle Tom's Cabin is a novel written by Harriet Beecher Stowe in 1852 - only a few years before the start of the Civil War. It described the evils of slavery and proved just how horrific it was.
It was so important because of the effects it had on society.
So, it didn't prove slavery wasn't bad (A), slavery in the North was not common or discussed in the book (C), and it didn't convince people that the North was evil (B).
However, it did show people a different and horrific side to slavery (B), laying some groundwork for the civil war.