B- quota system
It was included in the Emergency Quota Act of 1921, a temporary legislation in issued by the U.S. Government to limit the number of immigrants from certain countries. The law annually provided immigration visas to 3% of the total number of people of each nationality living in the U.S., based on the 1910 national census.
Though it officially restricted immigrants from all countries, it actually discriminated people from southern and eastern Europe or other non-European countries. And People of Northern Europe, who had similar cultures with Americans were benefited the most: They had a higher quota and were more likely to be admitted to the U.S.