What business practice contributed most to Andrew Carnegie’s ability to form a monopoly? using profits to support charities in order to greatly improving his reputation
increasing his profits every year
focusing on a single aspect of steel production
combining his companies into one company, and controlling all aspects of steel productiont.

combining his companies into one company, and controlling all aspects of steel production

Explanation:

By combining all of his companies into one company, Carnegie managed to obtain a huge advantage in the marketing point of view. He manage to ensure that his company is the only name that consumers would recognize whenever they're considering to buy steel products. This will make it really hard for new competitors to enter the market.

By controlling all aspects of steel production, he managed to ensure that the production of his product become really cheap and efficient.

All of the things above are the main reasons why Carnegie able to form a monopoly on the steel market and became the 3rd richest man in the world.

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