Select the letter-pair that can replace the question mark (?) in the following series. RJ, XP, DV, JB, ?

QH PI QJ PH

PH

Explanation: RJ, XP, DV, JB, ? The logic followed here is : R → (+6) → X → (+6) → D → (+6) → J → (+6) → P   J → (+6) → P → (+6) → V → (+6) → B → (+6) → H   So, the letter-cluster that replaces the (?) to complete the series is "PH"