Which of the following letter-number clusters will replace the question mark (?) in the given series to make it logically complete? EJ23, FK34, GL45, HM56, ?

Select the letter-cluster that can replace the question mark (?) in the following series: QZR, NYO, KWL, ?, EPF

Which of the given letter-number clusters will replace the question mark (?) in the following series to make it logically complete? USO 48 NLH 39 GEA 30 ZXT 21 ?