Which company was a monopoly during the gilded age

A company that had a monopoly during the gilded age includes:
-Rockefeller & Standard Oil (he was able to cut down his oil prices so much that competitiors failed to make money, and were out of business
- J.P Morgan & Banks (he owned one of the leading banks in the US and he bought out Carnegie Steel so that he could make his own steel. He also had a big influence on the US government after he helped bail them out in the Depression of 1895 and the Great Panic of 1907
- Carnegie & Steel (He used new technology like the Bessemer Process (which made stronger, more reliable, and cheaper steel) and made the largest steel company in the world

These are just some of the few "large" companies that had monopolies on certain useful resources 

hope this helps