Answer:
1. B. The United States declared the Rio Grande River as the border between Texas and Mexico.
Under President Polk, the United States claimed the Rio Grande river as the border with Mexico because he claimed that Texas claimed this to be their border with Mexico and since Texas was now a part of the United States, that was the U.S. border as well. This was after the Texas annexation in 1845.
2. C. Congress formally declared war on Mexico.
Congress formally declared war on Mexico on May 13, 1846 after both nations refused to back down from their contrasting claims as to where the U.S. - Mexico border was.
3. A. The Free Soil Party was organized to exclude slavery from Western territories.
The Free Soil Party was founded on August 9, 1848 to oppose the expansion of slavery into the Western territories. They were dissolved into the Republican party 6 years later.
