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歲以下的公民子女沒有排期. 21歲以上的公民子女才是F1, 與F2A(綠卡配偶, 未婚21歲以下的綠卡子女是兩回事.)