Direction: Which one set of letters when sequentially placed at the gaps in the given letter series shall complete it.
a_b_ba_b_ ba

ABC, FGH, LMN, ____

X, Q, K, F, ?  

bc, cde, de, efg, fg, ? 

deb, ijg, nol, ? , xyv 

QAR, RAS, SAT, TAU, ? 

V, VIII, XI, XIV, ? , XX 

CFL, EIK, GLJ, IOI, ?