父母曾经有绿卡，大概有十年左右，后来放弃了，已在中国至少六年。但现在又想来美，怎么办才最好？
-They can enter US using B2 visa, then you can sponsor them to get the green card within US as you did before. Another choice is that you may file I-130 for them to apply for the immigrant visa in the US Consulate in Guagzhou. Thus, they can be the green card holders using the immigrant visa to enter US.
In either cases above, they need to provide the green card abandonment proof, i.e. the approved document USCIS or the US Consulate gave/sent them for the green card abandonment such as the copy of I-407 endorsed by USCIS or the US Consulate.
