Following the Civil War, Why Were the Thirteenth, Fourteenth, and Fifteenth

Following the Civil War, why were the Thirteenth, Fourteenth, and Fifteenth Amendments added to the Constitution?

A) to guarantee African Americans seats in Congress
B) to move the nation toward a unitary government
C) to ensure that the Union's views on states' rights were the law of the land
D) to ensure that northern states would dominate the national government