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.

Q: hotel : taxi. airport : ?

A: train
****
Q: airport : train. airport : ?

A: train
****
Q: australia : airplane. island : ?

A:
boat
****