When World War I broke out in Europe in 1914, industrialized urban areas in the North, Midwest and West faced a shortage of industrial laborers, as the war put an end to the steady tide of European immigration to the United States
Huh English please then I can help
Dr. Smith is conducting a longitudinal case where it is a case being done in a same manner over a long period of time. It could be seen above as Dr. Smith has instructed his representatives to go back every five years to conduct the same measurement in a course of a long period which is until they are sixty years old which could be classified as the longitudinal case.
If you could only select one answer, my guess would be education and prevention.
As humans dispersed from the equator peoples skin got lighter and lighter because of fewer exposure to sun radiation.
They didn't get tanned as much :)
