If R = {(x, y) : x, y ? Z, x² + 3y² ? 8} is a relation on the set of integers Z, then the domain of R^-1 is

(a) {-2, -1, 1, 2}

(b) {0, 1}

(c) {-2, -1, 0, 1, 2}

(d) {-1, 0, 1}