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.

[EX Q]: hotel : taxi. airport : ?
[EX A]: train

[EX Q]: mainland : boat. downtown : ?
[EX A]: subway

[EX Q]: portland : car. mars : ?
[EX A]:
spaceship