Late comers got caught

This is a story of four friends who like most Indians were studying engineering. It was their final term examination the next day, and like everyone else they had put in a last night effort only to find out that they had completed no more than 30% percent of the syllabus. So forgetting everything they decided to take a drive and later come back and face the music. During this they are struck with the “BRILLIANT” idea of not appearing for the exam.

So the very next day all four of them go up to the professor and make this very commendable excuse that they had just gone for a drive to clear their mind and due to a punctured tyre, they could not return back in time. The professor satisfied with the statement, agreed to take a re-test and scheduled it on the upcoming week.
Very happy that they had fooled the professor the friends go back to their room and put up a Herculean effort for one week to complete off the whole syllabus well before time and finally they appear for the exam.

Before entering they meet the professor who is standing just outside the hall. ‘I have a few conditions’, he says.’You will be each given one question to answer,however everyone will be sitting in different rooms’.
The condition seemed fair enough and moreover being overjoyed by the fact that they had to answer only one question, they happily obliged.
The professor sent them to different rooms with the respective envelope containing their question papers. A few minutes after settling down they were asked to open the question paper.
To utter shock,
The question on the paper read “Which Tyre?”