Set Theory: subsets

Hi there, heres the question im given, i will provide the answer that I think is correct, do you mind checking it and possibly pointing out where im wrong if I am?

Give an example of a set S such that:
a) S is a subset P(N)
b) S belongs to P(N)
c) S belongs to P(N) and |S|=5

a) {1,2,3}
b) {{1}}
c) {1,2,3,4,5}