Direction: What should come in place of the questions mark (?) in the following letter series?
a, k, e, o, i, s,  ? __ ? _ , q , a , u , e 

Direction: Which one set of letters when sequentially placed at the gaps in the given letter series shall complete it.
ab_bba _ _bba _abb _ _a 

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, ?