Z, X, S, I, R, R,?,?

The given sequence is a combination of two series :

I. 1st, 3rd, 5th, 7th, 9th, 11th terms i.e. A, B, C, D, E, ? II. 2nd, 4th, 6th, 8th, 10th terms i.e. B, D, F, H, ?

Clearly, I consists of consecutive letters while II consists of alternate letters. So, the missing letter in I is F, while that in II is J. So, the missing terms i.e. 10th and 11th terms are J and F respectively.

The given sequence is a combination of two series:

I. C, F, I, L, O, ? and II. Z, X, V, T, ?

The given sequence is a combination of two series :

I. Y,T, O and II. B, G, ?

I consists of 2nd, 7th and 12th letters from the end of the English alphabet, while

II consists of 2nd, 7th and 12th letters from the beginning of the English alphabet.

So, the missing letter in II is the 12th letter from the beginning of the English alphabet, which is L.

The number of letters in the terms of the given series increases by one at each step.

The first letter of each term is two steps ahead of the last letter of the preceding term.

However, each term consists of consecutive letters in order.

The given series may be divided into 2 groups :

I. A, B, C, D, E, F, ?, ? and II. N, O, P, ?

Clearly, the given series consists of two terms of I followed by one term of II.

The missing terms in I are G and H while the missing term in II is Q.