On May 12, 1846, the United States Senate voted 40 to 2 to go to war with Mexico. President James K. Polk had accused Mexican troops of having attacked Americans on U.S. soil, north of the Rio Grande. But Mexico claimed this land as its own territory and accused the American military of having invaded. The Treaty of Guadalupe Hidalgo was signed in 1848, the treaty allowed the United States to purchase California, Arizona, New Mexico, Texas, Nevada, Utah, and Colorado for fifteen million dollars, doubling the size of the United States, but also displacing millions of Mexican citizens in new American territory. Mexico also gave up all claims to Texas and recognized the Rio Grande as America's southern boundary.
"<span>Social stratification developed based on newly specialized jobs" is the best option from the list, since this was due to the recent ability to domesticate plants and animals.</span>
