Select the letter-cluster that can replace the question mark (?) in the given letter-cluster series. YM, WO, UQ, ?, QU

TS SS ST RS

SS

Explanation: YM, WO, UQ, ?, QU The logic followed here is : Y → (-2) → W → (-2) → U → (-2) → S → (-2) → Q M → (+2) → O → (+2) → Q → (+2) → S → (+2) → U    So, the letter-cluster that replaces the (?) to complete the series is "SS"