Answer:
Oregon was a vast territory first explored by the Spanish and the French in the 17th and 18th centuries. In the 1830s, American pioneers traveled along the Oregon trail; joint settlement by British and Americans started. In 1846, a formal border with the British territories was fixed at the 49 parallel. Oregon was formally admitted into the Union in 1859.
Explanation:
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The answer is A. The Appalachian mountains along with the Proclamation of 1763 stopped colonists from settling west.