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

abCD, fgHI, ?, stUV, abCD

lmNO

Explanation:

abCD, fgHI, ?, stUV, abCD

The logic followed here is :

a → (+5) → f → (+6) → l → (+7) → s→ (+8) → s

b → (+5) → g → (+6) → m → (+7) → t → (+8) → b

C → (+5) → H → (+6) → N → (+7) → U → (+8) → C

D → (+5) → I → (+6) → O → (+7) → V → (+8) → D

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