Before we begin, let us differentiate between two things:

* The shape of a sphere,
* and the topology of a sphere.

A NURBS surface can make the *shape* of a sphere. In a typical configuration, it will be 'open' at the poles. That is the mathematical function of the surface does not wrap over the pole in that its is not a true sphere (it has the shape of a sphere). Topologically the sphere is in this typical configuration a cylinder. 

It is possible to leave the other direction open too in which case it is topologically a plane. Several CAD applications choose this approach. A torus is also possible if you allow a thin no volume sliver at the center of your sphere.

[![enter image description here][1]][1]

**Image 1**: Turning cylinder to sphere. Note this is not a topological sphere it is still a cylinder as the top is open (even tough infinitesimally small).


> Is the sphere homeomorphic to one of the above surfaces ?


No, but you can still have a spherical shape even if it does not satisfy the topology condition of mathematics.
### Why only 3 topological families?

Simply a NURBS surface has only 4 possible configuration of wrapping around the parameter space:

* It does not wrap at all. Topology: Plane.
* It wraps around the U direction. Topology: Cylinder.
* It wraps around V direction. Same as above. Topology: Cylinder.
* It wraps around both U and V. Topology: Torus.

A wrap is always periodic so it goes from - direction to + direction. It can not arbitrarily connect  (on a mathematical level).


  [1]: https://i.sstatic.net/kP30Z.gif