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> The reason stated is we failed to submit a qualified I-864 which we did, maybe the income of me is not high enough.

他们没有给你RFE就直接把你拒绝掉了吗？

> So at this point, what could we do? If we can find a joint sponsor for finacial support, do we need to re-file I864? pay fee again? 

joint sponsor需要填一份I-864。如果你是递交I-290B，不需要交再I-485的费用；要交I-290B的费用。如果重新递交I-485就要交I-485的费用。

> Should we include all joint sponsor's I-864 required files/info in I290B when we fill? 

是

> For I-290B part 2, which option should we check? 2a, reopen? 2b reconsider? or 2c reopen and reconsider?

应该是reopen，因为你是提供更多材料

> Does my mom still need to leave US within 33 days after we file I290B or she can wait in US till the decision is made?

不要离开。在美国等。