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They are in China.

I received two letters, one for my father and one for my mother requesting the same additional information. I only have one original of my parents' marriage certificate. 
-Fine to use the photocopy for your mother.

 Also, I got divorced years ago in China. They ask me to submit both prior and current marriage certificates. I Don't have prior marriage certificate, but a court order for divorce. Can I send it to them instead of prior marriage certificate?
-Yes, and explain you lost your prior marriage certificate.

Again, I only have one original.
-Provide it and your parents can get it back when they are interviewed in China.

 Thank you so much for all your help!