Why Was the Civil War an Important Event in the Development

Why was the Civil War an important event in the development of the American political system?

A) It revealed how differently the North and South interpreted the Constitution when it came to the relationship between the states and the federal government.
B) The leaders of the Confederacy relied on a view of American federalism that held that the country was created by agreement among sovereign states.
C) The leaders of the Union believed the Constitution meant the United States is a sovereign country, and the preamble declared that "the people" had created an indissoluble union.
D) Approximately 620,000 Americans died during the war; one out of every 10 able-bodied men in the North and one out of four in the South were killed or injured.
E) It settled the issues of secession and slavery, and emphatically shifted the country away from a federal system.