Mexico had an independence war against Spain and wanted to become an independent state. When it achieved its independence, they opened their northern borders to trade with the United States. The other countries don't border with United States, except for Canada which wasn't Spanish and doesn't border in North.
The correct answer for 2 is <span>a. clear
A manifest is something that is presented in a clear way and that is easily observed by anyone. In the United States, the famous Manifest Destiny was about making it clear that it is the Destiny of the United States to spread from one ocean to the other. That's why they moved westwards and took California and similar states from Mexico.</span>
By them moving to the frontier, they were advertising how they wanted to move there. They would show people the amazing aspects of the rest of the country, inspiring them to move there as well.