Why did Marxists come to believe that this distinction was paramount for understanding history and shaping the future?

Answer to the following question is as follows;

Explanation:

Capitalism, according to Marx, contained the germs of its own demise. He explained how the bourgeoisie's riches was dependent on the proletariat's labour. As a result, capitalism necessitates the existence of a working class. However, Marx foresaw that continuing to exploit this lower class would result in widespread discontent.

A rebellion against the capitalists would eventually be led by the proletariat. Capitalism and its followers would be overthrown in the ultimate battle.

can someone me with this question:  

1) the end of the conflict between the north and the south didn’t just mark the end of a war; it marked the end of an entire way of life.” which statement best describes what this statement means?  

a) the southerners were no longer free and were controlled by the northern states.

b) the industrial economy in the north was not sustainable.

c) slavery became illegal, which changed completely the south’s labor force and the agricultural economy was destroyed.

d) the states decided to relinquish their independence and return to britain’s rule.