Which Tire?

These four friends were so confident that the weekend bedore finals, they decicded to go up to Dallas and party with some friends up there. They had a great time. However, after all partying, they slept all day Sunday and didn't make it back to Austin 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 Dallas for the weekend with the plan to come back and 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 seperate rooms and handed each of them a test booklet, and told them to begin. They looked at the first problem, worth 5 points. It was something simple about free radical transformation. "Cool," they thought at the same time, each one in his seperate 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?