Select the letter cluster that will replace the question mark (?) in the following series. QPO, SRQ, UTS, WVU, ?

AZY VWX YXW XVZ

YXW

Next alphabet in the series is : Q → (+2)→ S → (+2)→U → (+2)→ W  → (+2) → Y P → (+2) → R → (+2)→ T → (+2)→ V  → (+2) → X O → (+2) → Q → (+2)→ S → (+2)→ U → (+2) → W