How did the carnegie’s purchase of allegheny steel contribute to the formation of his monopoly?

Carnegie decided that he was going to be a capitalist who concentrates on one industry - the steel industry. He constructed his first steel mill in the around 1875. The profit he made from this steel mill allowed him to buy up other nearby steel mills. As Carnegie's empire grew, he bought up more of the competing steel mills. His purchase of Allegheny Steel contributed to the formation of his monopoly because it was one of his last major competitors. The definition of a monopoly is a company or enterprise that is the only seller of a certain product. By the time Carnegie had finished buying up his competitors, his company was the only company left in the steel industry. 

The answer is c because the law is facts