So they're optional, but they're very, very, very highly encouraged that you do these, because this is going to give you practice for the types of problems that are going to be on the exam.

虽然它们是选做的，但是它们非常非常，非常值得你们去做，因为这对你们来说是一次训练,是用来熟悉考试题型的。

麻省理工公开课 - 化学原理课程节选

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

We're also posting a practice exam for you to take, so after you're completely done your studying, it's good to have everything done before you take the practice exam, and then sit down just with this sheet here and your calculator, and ideally a timer, and make sure you can do the practice exam in the allotted amount of time.

我们还将给出一份模拟试题，因此在你完全复习好之后，最好先复习完,再做这份模拟试题，坐下来,只拿出这张表与计算器，最好还有个计时器，确保自己完成模拟题,在规定的时间内。

麻省理工公开课 - 化学原理课程节选