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家里孩子小没人照顾，周五入境，周六寄RP材料，周日出境，再回来打指纹，这样操作有什么问题吗？
-Yes, there will be a big problem. Because it takes several days for USCIS to receive the materials and put them into the system and if you leave US before USCIS put your case into the system, then USCIS will consider that you send your application materials when you are physically present outside US. Thus, the application is invalid. So, you had better wait for a week after sending out the materials.

另外我材料都没寄到我就出境了，从技术层面，移民局怎么知道我是不是在境内寄的材料呢？
-After you leave US, USCIS will know you are not in US.