Which letter will replace the question mark (?) in the following series? CD, FH, IL, LP, OT, ?

QX RX SW TV

RX

According to the above alphabet position: CD ⇒ (3) (4) FH ⇒ (6) (8) IL  ⇒ (9) (12) LP ⇒ (12) (16) OT ⇒ (15) (20) ?   ⇒ (18) (24) ⇒ RX (add 3 in 1st alphabet position and 4 in 2nd alphabet position)