I-140 填表问题

Really hope some one can help me about this I-140 filingissue. Thanks a lot! The fact is thatwe have filed NIW petition and want to file EB1a . iIn the I-140 form part 4, there is a choice "Has any immigrant visa petition ever been filed by or on behalf of this person? if Yes-Attach an explanation", I assume I need to put yes, what explanation I need to attached? NIW petition receipt or something else?
Greatly appreciate any help!