Question 899800
ABCD_E_FGH_I_JKLMN_OPQRSTUVWXYZ
    
Looks like odd value long strings (1,3,5,7,...) then skipping 1 value,
ABCD_E_FGH_I_JKLMN_O_PQRSTUV_WXYZ
So the next string would be,
PQRSTUV