The correct option is (B) Immigration.
Because of immigration U.S. population continue to grow even though the United States has essentially established a zero population growth (ZPG).
Immigration is the international movement of individuals to a country of destination where they are not natives or citizens in order to establish themselves as long-term residents or naturalized citizens.
When a population is in demographic balance, or when the number of births plus in-migrants matches the number of deaths plus out-migrants, there is no population growth or fall. This state is frequently referred to as ZPG.
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