Get your lie straight

Four friends went out and partied the night before the final exam. The next morning they overslept and woke up in a panic because they had missed the exam and would fail the class.
One of them came up with the idea to tell the professor that they had a flat tire. So they told him this lie and he was sympathetic and let them take the final exam.
The last question on the exam was, “which tire was flat?” Of course each gave a different answer and failed