What was challenging about settling the land west of the Mississippi R

What was challenging about settling the land west of the Mississippi River in the late 1800s?

a. The region got little rainfall, and there was little lumber available for housing.
b. The region was frequently flooded by its rivers, which made farming it difficult.
c. The Great Plains had many deserts, and this made travel and farming difficult.
d. The region had various Native American groups who would not leave their land.
e. The Great Plains were known for their severe and unpredictable weather patterns.

ANSWER
A