What made standard oil a horizontal integration monopoly?

Standard Oil became a horizontal integration monopoly because it owned ninety percent of US oil refineries.

In 1870, John D. Rockefeller and Henry Flagler established the well known American oil producing, transporting and refining company which eventually became a monopoly in Ohio. It became a monopoly because it bought almost all of the competitors so that it managed to control almost all oil production, processing, marketing, and transportation in the United States.

i really really really really really really dont know