Question

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

Answer 1

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.