Which of the following sets are not convex?
(a) {(x, y): 8x² + 6y² ≤24)
(b) {(x, y): 6≤ x² + y² ≤ 36}
(c) {(x, y): y≥ 3, y≥ 30}
(d) {(x, y):x² ≤ y}