In What Situation Should a Negotiator Resort to Threats

In what situation should a negotiator resort to threats?

A) in cases where concessions are less sizable than desired
B) when the negotiator knows the other party is in a position where they cannot refuse
C) all the time and frequently as people respond to threats on a visceral level and thus are more likely to comply with demands
D) as a last resort when the other party refuses to budge
E) threats should never be used in negotiation