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.

washington : train. mall : ?
bus

mall : bus. downtown : ?
subway

airport : train. downtown : ?

subway