c. by buying all the other steel mills Explanation:
The Bessemer Process
This is a process known as horizontal integration.
Andrew Carnegie also created a vertical combination, an idea first implemented by Gustavus Swift. He bought railroad companies and iron mines. If he owned the rails and the mines, he could reduce his costs and produce cheaper steel.
Bad economic times and fear of another terrible war made European countries at the mercy of a political bully and opportunist like Hitler The rise of Communism and Fascism played on fears for the future.