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What does the f 1 category of American immigrants A stand for?

Adult unmarried children of American citizens and their minor children. Family immigration category: F 1: adult unmarried children of American citizens and their minor children. F2A: Spouses and underage unmarried children of permanent residents in the United States. F2B: Adult unmarried children of permanent residents in the United States. F3: Married children of American citizens, and their spouses and minor children. F4: Brothers and sisters of American citizens (age 2 1), their spouses and minor children. Adult: 2 1 year.