Select the letter-pair that can replace the question mark (?) in the following series. MR, NU, OX, PA, ?

RE QC RC QD

QD

Explanation: MR, NU, OX, PA, ? The logic followed here is : M → (+1) → N → (+1) → O → (+1) → P → (+1) → Q   R → (+3) → X → (+3) → X → (+3) → A → (+3) → D  So, the letter-cluster that replaces the (?) to complete the series is "QD"