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It seems the IO who dealt with your case is very low quality and inexperirenced. He/she should have checked the Application Type you chose in I-485 carefully first, then determined your qualification on I-485 filing. Application Type "b" obviously indicates that your I-485 filing is based on your spouse's green card approval as your spouse's derivative (i.e. using following to join).  It is very  headache to have such an IO to deal with your case.