What was the core business that made Standard Oil a horizontally integrated monopoly? A-finding new uses for oil. B-refining oil. C-building oil pipelines. D-transporting oil to customers.

the core business that made standard oil a horizontally integrated monopoly was : B-refining oil Back then, standard oil managed to gain monopoly within the oil industry by buying all of the existing rival refeneries and united it under the standard oil brand. This allow them to dominate the whole oil product that distributed in united states