Two analogies that relate actions to the tools used to perform the action is given in the form "A : B. C : ?". "A : B" relates action A to tool B. Your task is to replace the question mark (?) with the appropriate tool for the given action C, following the "A : B" relation.
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Question: crack : nutcracker. iron : ?

Answer: iron


Question: cut : knife. dry : ?

Answer: cloth


Question: dig : shovel. unlock : ?

Answer:
key