父母申请绿卡, file I-130 together or seperate?

父母申请绿卡, file I-130 together or seperate?

According to
http://blog.wenxuecity.com/blogview.php?date=200801&postID=6735,
it sames that I can file both parents together. but on I-130 instructruction, if only says 1(C), 1(D), or 1(E), separate petitions are not required for his or her hu*****and or wife. But parents are und 1(F).

Can anybody clarify this?

Thank you.