Students in a 100 Person Class Agree Before Hand to Answer

Students in a 100 person class agree before hand to answer all 20 multiple choice questions with B. Under which of these grading methods is this likely to prove an effective agreement that does not produce cheating on the agreement (for example, by a few students answering the questions correctly) ?

A) The professor gives the top five papers a 100, the next five best an 80, and the rest a 50.
B) The professor gives the highest score a grade of 100 and starts reducing scores by 5 points for every one wrong.
C) The professor substracts 5 points from 100 for every question wrong.
D) The best paper gets an A, the rest an F.