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I am over 21, but I am single. From immigration law, my parents can sponsor me to immigrate to USA.

-You are your parents's relative, not their immidiate relalive, so you are not qualifed for I-601A benefits.

See the following at https://www.uscis.gov/i-601a (note: the new rule also extend I-601A benefits to the immediate relative of a green card holder):

To file Form I-601A:

• You must be the beneficiary of an approved immigrant visa petition classifying you as an immediate relative of a U.S. citizen. An immediate relative is an individual who is the spouse, child (unmarried and under 21), or parent of a U.S. citizen.