First preference: Unmarried, adult sons and daughters of U.S. citizens (adult means 21 or older.)
Second Preference (2A): Spouses of Green Card holders, unmarried children (under 21) of permanent residents
(未婚21岁以下的公民子女没有排期. 21岁以上的公民子女才是F1, 与F2A(绿卡配偶, 未婚21岁以下的绿卡子女是两回事.)