The correct answer is "A".
The development of the Oil Industry during the early decades of the 20th century led to making Texas one of the most powerful states in the country. The discovery of the Texas Oil field in 1930 by Columbus Marion Joiner laid the path to an industry that is responsible for the development of very prosperous urban areas, such as the city of Dallas.
These urban centers, which were once settlements, experienced great growth as oil operations demanded services such as banking, groceries, and health. These needs made it possible for enterprises who operated in these fields move to these places and therefore, bring many people to the area, as many job openings were held.