Which most accurately describes how economic incentives led to the development of boom towns in the american west?

Growth in boomtowns is nearly always attributed to the exploitation of a natural resource for the instant gratification of workers looking to make a quick buck. Whether it was gold or oil, or in the latter days of the 20th Century, the building of real estate near desirable climates, the sudden development of a city around a previously worthless place is that makes the boomtown so attractive to the initial investors and workers, and those who follow them to provide services and commerce that support the commercial infrastructure of a city.