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 am pretty sure the awsers is d.

states

explanation:

as the old saying goes, all politics are local. state and federal governments affect our daily lives in numerous ways. yet, in our federal system, there is supposed to be a balance between federal and state power. as james madison envisioned it in federalist no. 51, “the power surrendered by the people” would be “divided between two distinct governments,” creating a balance of power that would enable the “different governments [to] control each other.”