Question

What made Standard Oil a horizontal integration monopoly?

It controlled all aspects of oil production.
It owned ninety percent of US oil refineries.
It formed a trust.
It operated all across the United States.

Answer 1

It owned ninety percent of US oil refineries.

Answer 2

It owned ninety percent of US oil refineries: Smaller competing oil companies couldn't compete with Standard Oil, so they were practically forced into selling their business and resources to Rockefeller.