Find the missing letters:

$ab, ba, abc, cba, abcd, ?$

Find the missing letters:

$FLP, INS, LPV, ?$

Find the missing letters:

?, SIY, OEU, KAQ, GWM, CSI

What should come in the place of the question mark (?) in the following letter series based on the English alphabetical order?

YXZ, XWY, ?, VUW, UTV, TSU

Find the missing letters:

?, PSV, EHK, TWZ, ILO

Find the missing letters:

PTVX, AEGI, ?, WACE, HLNP

Find the missing letters:

A, C, A, E, A, G, A, I, A, ?

Find the missing letters:

M, N, O, L, R, I, V, ?

Find the missing letters:

Z, W, S, P, L, I, E, ?