Which letter will replace the question mark (?) in the following series? AY, EU, IQ, MM, QI, ?

UE XF XE WC

UE

According to the above alphabet position: AY ⇒ (1) (25) EU ⇒ (5) (21) IQ ⇒ (9) (17) MM ⇒ (13) (13) QI ⇒ (17) (9) ? ⇒ (21) (5) ⇒ UE (Add 4 in 1st alphabet position and Subtract 4 in 2nd alphabet position)