For each ring R and subset S given below, determine whether S is an ideal in R.
(a) R = Z
S = {even numbers}
(b) R = Z
S = {odd numbers}
(c) R = C
S = R
(d) R = {functions from R to R}
S = {f ∈ R | f(3) = 0}
(Note: This is not the same as the ring of polynomials with coefficients in R. For example, e^x is a function from R to R, but it isn't a polynomial.)