Answer:
They introduced new business practices that created large industries and produced great wealth for a few.
Step-by-step explanation:
During the 19th century, large companies in America tried to become monopolies and extremely wealthy and powerful individuals like Rockefeller and Carnegie (among others) succeeded in creating them.
At first the general public benefited from them, since large companies became monopolies by selling their products or services at very low costs driving their competitors out of business. Once their competitors where under severe financial stress, they would acquire them, so these large monopolies grew through horizontal integration.
At the same time, trusts were established so that they could purchase smaller companies in order to vertically integrate their whole supply chain management process. E.g. these trusts ended up buying railroads, factories.
Once the monopolies were consolidated, consumers were forced to pay high prices for goods that they needed on a regular basis. Their extreme market power had to be regulated, and president Harrison ended passing the Sherman Antitrust Act of 1890. But it wasn't until president Theodore Roosevelt came that the law was really enforced since monopolies and trusts didn't only have a large market power, they also had a huge politician buying power.