Trick Question

This past fall semester, at Duke University, there were two sophomores who were taking Organic Chemistry and who did pretty well on all of the quizzes, midterms, labs, etc. Going into the final exam, they had solid "A's."

These two friends were so confident going into the final that the weekend before finals week (even though the Chem. final was on Monday), they decided to go up to University of Virginia to a party with some friends.

So they did this and had a great time. However, they ended up staying longer than they planned, and they didn't make it back to Duke until early Monday morning. Rather than taking the final then, they found Professor Aldric after the final and explained to him why they missed it.

They told him that they went up to Virginia for the weekend, and had planned to come back in time to study, but that they had a flat tire on the way back and didn't have a spare and couldn't get help for a long time. So they were late getting back to campus.

Aldric thought this over and agreed that they could make up the final on the following day. The two guys were elated and relieved. So, they studied that night and went in the next day at the time that Aldric had told them.

He placed them in separate rooms, handed each of them a test booklet and told them to begin. They looked at the first problem, which was something simple about free radical formation and was worth 5 points. "Cool" they thought, "this is going to be easy." They did that problem and then turned the page.

They were unprepared, however, for what they saw on the next page.

It said...

(95 points)
"Which tire?"