Four pairs of words are given here out of which the words in three pairs bear a certain common relationship. Choose the pair in which the words are differently related.

Stamp : Letter Ticket: Train Ink : Pen Car: Engine

Car: Engine

Stamp is pasted on the letter; ticket is bought to travel by train, ink is put inside the pen whereas, in the last option the latter one is part of the former i.e. engine is inside the car.