In the early decades of the nineteenth century, "mountain men" such as Jedediah Smith and Jim Beckwourth explored the western United States and earned a living by trapping animals for their fur. How did they contribute to westward expansion?

A. They established outposts that would later become towns.

B. They recruited workers to build the transcontinental railroad.

C. They surveyed land so it could be sold to farmers.

D. They blazed trails that later pioneers would follow.