What Makes a Grim Trigger Strategy "Grim" Is

What makes a grim trigger strategy "grim" is

A) if one player overprices, then the other overprices to the point of zero quantity demanded.
B) if one player underprices, then the other player notifies the Federal Trade Commission.
C) if one player underprices, then the other player is driven out of the market.
D) if one player underprices, then the other player drops the price so far that profits for both firms are zero.