Answer:
Not a function.
Explanation:
Due to the fact that an element from the first set of components is associated with one and only one element from the other. In this case, as it is a circumference, the elements match with more than one element of the other set. I'll give you an example.
Note: I've tried to keep it as simple as possible, and I'm not natively English so I don't really know how some things are mentioned, but I've also made a picture representation and I think it simplifies.