What makes an undertaking a dominant undertaking? To what extent may refusal to supply by a dominant undertaking constitute an abuse of dominant position?

Question:

1. What makes an undertaking a dominant undertaking? To what extent may refusal to supply by a dominant undertaking constitute an abuse of dominant position?
2. What importance does the four freedoms of European Union have on union law? Explain this in relation on to harmonisation and in relation to the continuous development of the internal market.