What made it difficult to travel over western terrain

Western terrain, compared to the east, was very different. As people traveled westward, the environments they went through would change. Also, the Rocky mountains were a severe extreme for travelers. The Appalachians on the eastern side were a challenge in themselves, but the Rockies were steeper, colder, and taller.

It made it difficult because of the steep ascents and descents over the rocky terrain.

Ibelieve the correct answer is d.