When paleontologists refer to the "big five," to what are they referring? the five most influential organisms that have now become extinct. the five most influential mass extinctions in earth's history. the five most influential events that have changed the surface of the earth. the five most influential inventions that have allowed them to study mass extinctions

the first phase of the government’s plan for settlement was building the transcontinental railroad. the railroad provided a way to bring settlers and manufactured goods west and ship their agricultural and mining produce east. the transcontinental railroad was an essential artery for rapid development of the frontier.

the second phase of the government’s plan was a liberal land distribution policy that made it possible for many people to homestead. with these two key elements—transportation and cheap land—the government rapidly achieved its goal of persuading people to move west, settle on farms, and push back the frontier.

i think it's severe storms but i might be wrong