Final answer:
John D. Rockefeller had to work hard to succeed in business and build a fortune in the oil industry through aggressive tactics like horizontal and vertical integration.
Step-by-step explanation:
John D. Rockefeller had to learn to work hard because he came from a modest background and wanted to succeed in business. He started his career in the oil industry, refining crude oil into kerosene. He used aggressive business tactics such as horizontal integration and vertical integration to dominate the oil refining industry, driving out competitors and creating a monopoly. His hard work and ruthless methods helped him build a vast fortune and become one of the richest people in history.