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1） 我F1，和绿卡结婚，用AP卡入境，护照上显示DA的含义是什么？
It means "Advance parole granted by District Office". In other words, you used AP to enter US.

 2）我是否仍然需要保持F1身份？
-No more. Your F-1 was void after you used AP to enter US, but you can stay legally in US to continue your study as a parolee.