Hey, There! I believe the correct answer for this would be "B. a drastic decline in the American Indian population" during the westward expansion a number of american Indians were pushed of their land and violently oppressed. Many died from diseases and battles to protect their land, but mainly from disease.
D i think i hope this helps!
B. The Gadsden Purchase was the last major land acquisition prior to the Civil War. In 1853, the ten million dollar purchase made by U.S. ambassador James Gadsden completed the continental U.S. as we know it. Initially, the strip of land was viewed as necessary for the completion of a southern trancontinental railroad. Eventually, the Southern Pacific Railroad was constructed here.
Productive Recources. hope this is right