Which of the Following Factors Explains Why U

Which of the following factors explains why U.S.immigration rules and regulations are so often changed and adjusted?

A) concerns about declining population,especially in parts of the West and Northwest
B) efforts to maintain policies similar to those of other nations
C) efforts to maintain the country's status as a "melting pot"
D) war,racism,prejudices,changing ideologies and tolerances,and fluctuating needs for labor