The map represents the northwest region of the United States. A line extends across the region from east to west moving over the Rocky Mountains towards the coastal region of Oregon. Review the map above. Why is the black line significant in American history? (5 points)

a
It represents the northern boundary line for territory claimed by Mexico.

b
It represents the main route for settlers moving into the Oregon Territory.

c
It represents the northern boundary line for the territory claimed by the United States.

d
It represents the path of the Transcontinental Railroad, a major trade route in the United States