回複：父母持綠卡，在國內20個月，這周末闖關回美～～驚恐緊張中

If their answer to the question: how long did you stay in China, is 20 months, CBP officer by regulation must send them to secondary inspection for further questioning. And unfortuanately that is the must-ask question CBP should ask every GC or AP person. You parent should insist to be admitted and not choose to return to China voluntarily, that way they will most likely to be allowed in and given a notice to appear before a judge.