What business practice contributed most to andrew carnegie's ability to form a monopoly?

I think , what mostly contributed to Andrew Carnegie's ability to form a monopoly, was his particular strategy to acquire the sources of materials he needed in the industry. He did not have to depend on suppliers for raw materials, but instead he acquired the supply sources (iron ore fields, mines, oil fields, etc.)

They would be more crops at 1a and 1b more than 1c