Following the Civil War,why Were the Thirteenth,Fourteenth,and Fifteenth Amendments Added

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

A) to guarantee political representation for all African Americans
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
E) to protect northern industry and financial interests