Option 'B' = changes in climate, had the greatest impact on pre-historic man.
Changes on the weather forced men to move periodically or cyclically, usually returning to their original location at various times. When they moved to colder climates, they would make clothing out of animal skins, to protect themselves. When the nomads learned how to plant, they didn't need to search for other places anymore.
The best option in terms of the Gentleman’s Agreement of 1907 would be that "<span>The United States would not segregate Japanese immigrants, and Japan would not allow further emigration to the United States," although it had less to do with segregation and more to do with not allowing more Japanese into the US. </span>
