College Joke : Real Test

As final exams neared, two students, very confident of their A averages in Chemistry class, decided to spend a weekend enjoying the social life of a nearby college. Although their Chemistry final was the first thing Monday morning they were reasonably certain they could pull it off. After a very late Sunday evening they overslept and did not arrive back on campus until Monday afternoon. In the hopes of avoiding failing the exam the two decided to tell their professor that they had a flat tire on the way back to campus.
Sympathetic to the situation, the professor allowed them to make up the exam. After being seated in different rooms the two opened their exam books and began working.
The first question, for 25 points, was a simple question on fusion. When they turned the page to answer the next question, however, both students shared the same look of despair though they were seated in different rooms.
75 point question: Which tire was flat?