Instructions for I-765 would provide answers. c31 could be used, c9 for pending adjustment
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Either designation would be OK and should not be a reason for the delay. As of now, many biometric processing centers have been backlogged because of the DACA (Deferred Action for Childhood Arrivals). That may be the reason. If you are very concerned, you can request an infopass (infopass.uscis.gov) and see what is going on with your case. I wouldn't worry about it to much for now but if it takes more than 90 days, then I would make an infopass appointment or have your attorney schedule it. Hope this helps!
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