The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
The factor that most contributed to the population shifts shown in the list is the Dust Bowl that affected the United States plains during the 1930s.
This was a time in this region of the United States, in which drought and the lack of rain created dust winds that severely affected crops and killed livestock in places such as Oklahoma and other regions of the plains.
It was a very difficult tie for farmers and the American people that already had been affected by the Great Depression after the stock market crash of October 29, 1929.
That is why many people affected by the Dust Bowl decided to move west, to California, to start a new life.
