回複：誠請指教: 公民,想同時為父母申請綠卡 ...

try to get B visa and come here to file I-485, bring all documents.
It is easier to go through I-485 process in US if parents plan to stay >6 months in US for this B2 trip.

If they cannot get B2, then they have to do consular processing. Guanzou consulate may be difficult to deal with if your parents are not healthy or wealthy.