Select the letter-pair that can replace the question mark (?) in the following series. KP, HM, EJ, BG, ?

XC YD ZE YC

YD

Explanation: KP, HM, EJ, BG, ? The logic followed here is : K → (-3) → H → (-3) → E → (-3) → B → (-3) → Y   P → (-3) → M → (-3) → J → (-3) → G → (-3) → D   So, the letter-cluster that replaces the (?) to complete the series is "YD"