These won't be graded, you don't have to turn them in, it's just to give you some extra practice if you want while you're studying for the exam.

这些题目不会记分，你们也不需要交上来，只是如果你需要的话，这可以让你,在准备考试的过程中得到更多的训练。

麻省理工公开课 - 化学原理课程节选