What core business made Standard Oil a horizontal monopoly?

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.

Answer: B. Refining oil.

Refining oil was the primary industry around which Standard Oil was based as a horizontally integrated monopoly. John D. Rockefeller and Standard Oil sought to dominate the refined oil due to the ability to control as many refineries as possible throughout the United States, thus controlling the refining process entirely. Standard Oil utilized a two-step method of centralizing production and reducing cost, taking all companies they could acquire within similar products and then moving on to cutting prices on refining, thus creating efficiencies and greater production and supply chain control as a vertically integrated monopoly. Because they owned multiple refineries as opposed to ownership at differing levels of production, this horizontal integration gave Rockefeller and Standard Oil a means to control prices for refined oil and maintain enhanced control of supply chains. Refining was the primary business activity that enabled Standard Oil to maintain this approach for market dominance and sub-vertical uses to its business strategy.