Lewis and Clark's exploration of the Missouri River was very important for the westward expansion of the United States. For example, the information they gained A. prevented the British from expanding their settlements from the Pacific Northwest eastward.
B. allowed the river to become a major transportation route for the Army during the Mexican War.
C. prevented the Native American tribes of the Rocky Mountains from harassing American settlers.
D. allowed the river to become a major transportation route for settlers of the Great Plains.