Which of these is the BEST explanation for the declining percentage of foreign-born people in the United States between 1910 and 1940?

Adeclining U.S. birth rate
Bimproved economic conditions in Europe during the 1920s
Cmore restrictive immigrations policies in the United States
Dpost World War II conditions in both Europe and the United States

Verified Answer :

The best explanation for the declining percentage of foreign-born people in the United States between 1910 and 1940 would be "C) more restrictive immigrations policies in the United States," since there was a heavy "Nativist" sentiment in the US during this time. 

it is      C) more restrictive immigrations policies in the United States