Answer:
A. Railroads.
Step-by-step explanation:
Even though steel and oil were big parts of the business, railroads were the primary factor in excelling big business out in the economy. Railroads had helped transport oil throughout the economy and helped expand the economy out west. So, big businesses loved railroads since it could excel their business to greatness. It had provided a growing market throughout the whole country like never before, and that is why that is the best answer out of your list.