A professor stood before his class of 20 senior organic biology students, about to hand out the final exam.
"I want to say 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. To ensure no one's GPA is affected by potential over-celebration this week, anyone who opts out of the final exam today will receive a 'B' for the course."
Great rejoicing spread through the class as students got up, thanked the professor, and signed out on his offer. After the last accepting student left, the professor looked at the handful remaining and asked, "Anyone else? This is your last chance." One final student stood up and accepted.
The professor closed the door and took attendance of the remaining students. "I'm glad to see you believe in yourselves," he said. "You all have 'A's."