Answer: D: Carnegie realized that steel was the metal of the future and began to invest in or buy up potential competitors.
Step-by-step explanation:
Carnegie realized that the bridges and railroads would soon be replaced with steel ones, especially after discovering the Bessemer process. Because of this, Carnegie opened his first first steel plant. It was only a few years later that he invested in other companies to ensure that his company of steel would thrive.
Hope this helps :)