Which Robber Baron became one of the wealthiest people in the world by creating a trust that controlled most of the oil refineries in the United States

Trusts took control over like the whole industry - literally.

This could become a huge risk over other small businesses. They made them run out of money, which lead to a decline, and eventually, the business would close.

Trusts basically went like this:

A guy takes control over what the industries do. He makes sure that their is no competition and runs the other's outa business. Sorta has to do with a monopoly.

Robber Baron did something similar to what Rockefeller did. He took control by using a trust in the oil industry, and became wealthy.