This question is really ambiguous. Here is a source that somewhat explains the whole thing. Use "edit" on browser>"find"> then type sickle cell (under title codominance of journal article).
http://www.ncbi.nlm.nih.gov/books/NBK21226/
In some senses it is recessive, in others it shows incomplete dominance. I really don't like doing TBR questions, because they can get ridiculous sometimes.
We don't even know how HbC acts. Is this a stand alone question or is this part of a passage?