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.

island : boat. hospital : ?
ambulance

france : airplane. airport : ?
train

mainland : boat. mars : ?

spaceship