Which letter cluster will complete the given series?
UQFL, XNKG, AKPB, DHUW, GEZR, _____
U + 3 = X, X + 3 = A, A + 3 = D, D + 3 = G, G + 3 = J Q - 3 = N, N - 3 = K, K - 3 = H, H - 3 = E, E - 3 = B F + 5 = K, K + 5 = P, P + 5 = U, U + 5 = Z, Z + 5 = E L - 5 = G, G - 5 = B, B - 5 = W, W - 5 = R, R - 5 = M
