Final answer:
Pittsburgh, Pennsylvania became the steel and iron capital of the U.S. by 1900, thanks to Andrew Carnegie and the efficient production methods he introduced, like the Bessemer process.
Step-by-step explanation:
The city that became the steel and iron capital of the U.S. by 1900 was Pittsburgh, Pennsylvania. This was largely due to the influence of Andrew Carnegie, who, after observing the Bessemer process in England, brought this efficient method of producing steel back to the United States.
Carnegie utilized vertical integration, buying out suppliers, to cut costs and control quality, which allowed him to dominate the U.S. steel market. By 1900, Carnegie Steel produced more steel than all of Great Britain, and Pittsburgh's access to raw materials like iron ore and coal, along with its robust manufacturing infrastructure, cemented its status as the center of American steel production.