Historians of the war can be divided into two schools. The first considers that it was the unavoidable outcome of conflicting 　1　between Northern and Southern 　2　. The second blames it 　3　political leaders for 　4　to avert an unnecessary war.
　　Analysts are also divided on whether the issue of slavery was the 　5　cause of the war, or a symptom of other, more 　6　differences—especially sectional interests and the doctrine of states ' rights—between the North and South which had been 　7　since the 　8　of the American republic.
　　The fundamental 　9　was economic. In the early 1840s the Northern states began the process of 　10　, modernizing their society to meet the demands of economic change. In       　11　, the slogan of Abraham Lincoln's 　12　party, “free labor, free land, free man” encapsulated the ideology of valuing the freedom of individuals to grasp the opportunity for economic self advancement in a booming, expanding society.
　　The Southern states remained stubbornly 　13　both economically and socially. It was a backward-looking way of life of tall white mansions on great 　14　dependent on a labor system which made slaves of approximately 4 million black Americans.
　　America was 　15　divided by economic structure, and was led into fratricidal warfare by a series of political clashes. The most common cause was the future of the West. The crises over California's admission in 1850 and over Kansas-Nebraska in 1854 were 　16　of the divergent economic interests of North and South in relation to the West.
　　The North wanted free land for independent labor in the same new territories where the South 　17　to perpetuate its traditional way of life by 　18　slavery. The issue was not the slavery already practiced, but the 　19　of its extension 　20　the West.
______

A.union

B.unification

C.formation

D.settlement