How to get an 'A'

A professor stood before his class of twenty senior organic biology students, about to hand out the final exam.
"I want to say that it's been a pleasure teaching you this semester. I know you've all worked extremely hard and many of you are off to medical school after summer. So that no one gets their GPA messed up because they might have been celebrating a bit too much this week, anyone who would like to opt out of the final exam today will receive a 'B' for the course."
There was much rejoicing in the class as students got up, walked to the front of the class, and took the professor up on his offer. As the last taker left the room, the professor looked out over the handful of remaining students and asked, "Anyone else? This is your last chance." One final student rose up and opted out of the final.
The professor closed the door and took attendance of those students remaining. "I'm glad to see you believe in yourself," he said. "You all get 'A's."
You have to be smart, don't you!