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: elect : president. swing : ?

Answer: sword


Question: sail : boat. break : ?

Answer: promise


Question: pet : cat. send : ?

Answer:
message