谢谢8老师的解答。我又去看了I-485的Instruction(https://www.uscis.gov/sites/default/files/document/forms/i-485instr.pdf)关于Marriage Certificate的一节(What Evidence Must You Submit with Form I-485?下第6项，在第11页上)，第一句话是：
If you are filing Form I-485 as the derivative applicant spouse of the principal applicant, you generally must submit a photocopy of your marriage certificate issued by the appropriate civil authority where the marriage took place.
似乎是说，如果是副申请人是配偶，就必须交婚姻证明。虽然没说，但是不是隐含是如果申请人是主申请人，就不用？第6项还列举了别的一些须要婚姻证明的类别，但应该不适用于公民父母绿卡申请吧？
