Which Letter will come in the place of question mark (?) ? Z, S, W, O, T, K, Q, G, ?, ?

N, C O , C N, D O, D

N, C

Z - 3 = W ,   W - 3 = T ,     T - 3 = Q ,   Q - 3 = N S - 3 = O ,    O - 3 = K ,      K - 3 = G ,   G - 3 = C