Find all cluster points of each of the following sets.
a) E = R\Q
b) E = [a, b), a, b ∈ R, a < b
c) E = {(-1)nn : n ∈ N}
d) E = {xn : n ∈ N}, where xn → x as n → ∞
e) E = {l, 1, 2, 1, 2, 3, 1, 2, 3, 4,...}