Which letter cluster will replace the question mark (?) to complete the given series?

ABCD, EBCD, ?, OBCD, UBCD

IBCD

Explanation:

ABCD, EBCD, ?, OBCD, UBCD

The logic followed here is :

A → (+4) → E → (+4) → I → (+6) → O → (+6) → U

B → (0) → B → (0) → B → (0) → B → (0) → B

C → (0) → C → (0) → C → (0) → C → (0) → C

D → (0) → D → (0) → D → (0) → D → (0) → D

So, the letter-cluster that replaces the (?) to complete the series is "IBCD"