Which Statement Provides the BEST Example of a Grim Trigger

Multiple Choice

Which statement provides the BEST example of a grim trigger rule?

A) A price increase by one firm is not met by the other firms in the market.
B) A decision by one firm triggers government regulation of all firms in the industry.
C) A negative decision triggers a tit-for-tat strategy.
D) A decision by one player to defect is met by a permanent retaliatory decision.