Two analogies that relate places/locations to the associated travel mode is given in the form "A : B. C : ?". "A : B" relates place A to travel mode B. Your task is to replace the question mark (?) with the appropriate travel mode for the given place C, following the "A : B" relation.
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Question: portland : car. office : ?

Answer: car


Question: work : car. mainland : ?

Answer: boat


Question: mall : bus. island : ?

Answer:
boat