Select the letter-cluster from among the given options that can replace the question mark (?) in the following series. BY, ?, DW, EV, FU

CE AX AU CX

CX

Explanation: BY, ?, DW, EV, FU The logic followed here is : B → (+1) → C → (+1) → D → (+1) → E → (+1) → F  Y → (-1) → X → (-1) → W → (-1) → V → (-1) → U  So, the letter-cluster that replaces the (?) to complete the series is "CX"