我可以填I864EZ表嗎？

看了一下說明，我給父母同時申請綠卡，似乎可以填I864EZ表，instead of I864表.  But I've got one question: Part 1 C: ;

The sponsored immigrant is the only person immigrating based

on the underlying visa petition

Should I answer Yes?  My parents are currently in the US on B2 visa.  What does the underlying visa refer to here?

Thank you!