Which letter-cluster will replace the question mark (?) in the following letter series? TWZ, VYB, XAD,?

ADG YBE ZBE ZCF

ZCF

Explanation: Explanation: TWZ, VYB, XAD,? The logic followed here is : T → (+2) → V → (+2) → X → (+2) → Z  W → (+2) → Y → (+2) → A → (+2) → C  Z → (+2) → B → (+2) → D → (+2) → F   So, the letter-cluster that replaces the (?) to complete the series is "ZCF'