Two analogies that signify affordances are given in the form "A : B. C : ?". Affordance is the possibility of an action being done on an object, for example book is an affordance of writing. The phrase "A : B" implies that B is an affordance of A. Your task is to replace the question mark (?) with the appropriate affordance of the given action C, following the "A : B" relation. Your answer should be a single object without further explanation.
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Question: play : game. wear : ?

Answer: coat


Question: operate : machine. eat : ?

Answer: food


Question: break : promise. write : ?

Answer:
letter