RFE Letter for birth cerficiate

I am applying green cards for my parents and received a RFE letter requesting additional birth certificate to demontrate the relationship of my parents and me. It stated that the notarized birth certificate is a secondary evidence. I read the notes in similar situation and the advise is to resend the notarized letter again.  I am wondering why they would accept it. Any help you can offer will be greatly apreciated.

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• 回复：RFE Letter for birth cerficiate -863211- ♂ (1614 bytes) () 12/02/2013 postreply 15:21:57

• 回复：回复：RFE Letter for birth cerficiate -CZH- ♀ (470 bytes) () 12/02/2013 postreply 15:27:53

• 回复：回复：回复：RFE Letter for birth cerficiate -863211- ♂ (928 bytes) () 12/02/2013 postreply 15:33:08

• 回复：回复：回复：回复：RFE Letter for birth cerficiate -CZH- ♀ (313 bytes) () 12/02/2013 postreply 15:40:34

• 回复：回复：回复：回复：回复：RFE Letter for birth cerficiate -863211- ♂ (551 bytes) () 12/02/2013 postreply 19:17:42