What Was the Primary Reason for Pioneers' Movement West

What was the primary reason for pioneers' movement West?

A) to settle an uninhabited expanse of land, introducing its first cultural traditions
B) to lead an isolated lifestyle devoid of capitalistic influences
C) to seek to improve their lot economically, such as by farming, mining, and hunting
D) to prevent the expansion of slavery because cotton was impossible to grow in new territories
E) to adopt and preserve Hispanic traditions that offered an alluring contrast to the eastern states