A customer of a competitor comes to you, how do you treat him?

C- Just like all other customers of yours ,  which is of course special 

C & A

B&C - For me each customer would be special (Either current/Potential). As a professional we should sell what customer's want rather than trick him for ones he doesn't & thus build rational for growth & development.

It also means the existing customer of our competitor is not satisfied with them thus they are exploring their opportunities with your company. Thus, even A applies here as we should be careful understanding their needs & than mention your company's products superiority & functionality which suits customer needs. :-)

Just like all other customers 

A, B and C.