Select the letter-pair that can replace the question mark (?) in the following series. JL, GJ, DH, AF, ?

YD XC YC XD

XD

Explanation: JL, GJ, DH, AF, ? The logic followed here is : J → (-3) → G → (-3) → D → (-3) → A → (-3) → X   L → (-2) → J → (-2) → H → (-2) → F → (-1) → D  So, the letter-cluster that replaces the (?) to complete the series is "XD"