Why did monopolies come about?

5  Congress enacted it in 1890 when monopolies were trusts. A group of companies would form a trust to fix prices low enough to drive competitors out of business. Once they had a monopoly on the market, they would raise prices to regain their profit. His monopoly allowed him to control the price of oil.

the answer should be a. the sui empire, the tang empire extended from korea in the west to manchuria in the east.