Final Exam

Kinda old but here it is...

THE FINAL EXAM
At Duke University, there were four sophomores taking Organic Chemistry. They did so well on all the quizzes, midterms and labs, etc., that each had an "A" so far for the semester. These four friends were so confident that the weekend before finals, they decided to go up to the University of Virginia and party with some friends there. They had a great time - however, after all the hardy-partying, they slept all day Sunday and didn't make it back to Duke until early Monday morning. Rather than taking the final then, they decided to find their professor after the final and explain to him why they missed it.
They explained that they had gone to UVA for the weekend with the plan to come back in time to study, but, unfortunately, they had a flat tire on the way back, didn't have a spare, and couldn't get help for a long time. As a result, they missed the final.
The professor thought it over and then agreed they could make up the final the following day. The guys were elated and relieved. They studied that night and went in the next day at the time the professor had told them. He placed them in separate rooms and handed each of them a test booklet, and told them to begin.
They looked at the first problem, worth five points. It was something simple about free radical formation.
"Cool," they thought at the same time, each one in his separate room, "this is going to be easy." Each finished the problem and then turned the page.
On the second page was written:
(For 95 points): Which tire?