How did john d. rockefeller horizontally integrate his monopoly in 1880?

a. he purchased iron mines around the country to add to his business.
b. he created a trust that controlled ninety percent of the nation's oil refineries.
c. he created a trust that controlled oil wells, refineries, and distribution networks.
d. he purchased coal plants around the country to add to his business.

It should be B. He created a trust that controlled 90% of the nation's oil refineries. If I remember correctly, horizontal integration is when you buy out the other businesses in the same market, while vertical integration is where you take control of everything related to your product, which is getting the oil, refining it, and distributing it, rather than selling those tasks out to third parties.