8. Which of the given sets of whole numbers is closed under addition? If the set is not closed, give an
example of two elements from the set whose sum is not
in the set.
(a) {10, 15, 20, 25, 30, 35, 40, ...}
(b) {1, 2, 3, . . . , 1000}
(c) {0}
(d) {1, 5, 6, 11, 17, 28, ...}
(e) {n EN n 2 19}
(f) {0,3, 6, 9, 12, 15, 18, ...}