D. because none of them explain the true meaning of what you want
The answer is the bill of rights
The use of horses in their defense and war strategy enabled the Native Americans to better resist Spanish encroachments. Being more mobile, faster and more able conduct attacks and raids made the Native Americans harder for the Spanish to oppose.
Nebraska was too far north for plantations--people of Nebraska wanted territory without slavery. Kansas was further south and platations could be built there. As soon as the Kansas-Nebraska Act was passed, thousands of people rushed to claim land in Kansas. This would tip the vote on slavery.
