Which statement best summarizes the process of the abolition of slavery in the United States?
A. The U.S. government decided to abolish slavery to follow the example of European powers.
B. Slave owners agreed to abolish slavery in order to prevent a slave rebellion like the Haitian Revolution.
C. The abolition movement gradually outlawed slavery using a series of legal challenges.
D. The abolition movement struggled for decades, but it took a war to achieve its goals.