回複：because u derived ur status

The question is similar to your case.

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Q: I am an alien and a U.S. citizen couple adopted my son ten years ago when he was 14. He later obtained permanent residence and then became a U.S. citizen based on the adoptive relationship with his U.S. parents. Now the adoption relationship is terminated legally and my son wishes to reestablish our natural parent-child relationship and to bring me to the U.S. May I immigrate into the U.S.?

A: No, You may not enjoy immigration benefit from your son based on a benefit that he derived from an adoption, even if there is a parent-child relationship between you.

http://www.hooyou.com/adoption/faq.html