Answer:
By around 1890, Rockefeller's Standard Oil Company controlled 90% of the oil refining business. Rockefeller made HUGE profits by paying his employees very low wages and driving his competitors out of business by selling his vast oil at a lower price than it cost to make it. Once he controlled the market, he would then jack up the prices. This is why he and other tycoons were known as "robber barons".
Source: Internet (Quizlet)