deb, ijg, nol, ? , xyv 

QAR, RAS, SAT, TAU, ? 

V, VIII, XI, XIV, ? , XX 

CFL, EIK, GLJ, IOI, ? 

BDF, ? , PRT , WYA 

XYZ, ABC, UVW, DEF, RST, GHI,_ ? 

AO, BM, CK, _ EG. 

AB, DEF, HIJK, ? , STUVWX