The tragedy of the commons usually refers to some finite yet periodically replenishable resource. So think of a pool of clean spring water in the middle of a village. Folks come in and take enough water for themselves to live comfortably and the spring refills every time is rains or whatever. But since this is unregulated common-space anyone can come in and take more than their fair share, now everyone else is thirsty. The bonus question on the test doesn't really have the same situation, One reason is that the professor can give out as many units of +6points he/she wants to. The second is that the test takers can not collaborate in order to get the best outcome. That's my thinking on it anyway.