So, in terms of practicing this weekend, I mentioned that instead of getting a problem-set today, what I am going to be posting is optional extra problems.

而对于这周的练习，我曾经说过今天将不会布置作业，但是会布置一些额外的习题。

麻省理工公开课 - 化学原理课程节选

You have a freewill offering--just because someone wants to do this, a freewill offering. And you have a vow offering that would be offered on the successful completion of a vow, for example. And these sacrifices are all entirely optional.

自由祭品--只因为有人想这么干，自由祭品，还有一个誓言祭品,举个例子，一个誓言如果成功达成，便会供奉祭品，这些祭品都是可选择的。

耶鲁公开课 - 旧约导论课程节选

The committee has suggested that it should only be optional for a two year transitional period, becoming compulsory thereafter for large cross-border companies, with small businesses being forced to join a common system after a further five years.

BBC: Corporate tax debate

The organization argues that a single, optional regulator would establish a uniform level of consumer protection, it would give consumers faster access to a broader range of insurance products and it could increase competition in the marketplace.

FORBES: Magazine Article

In addition, some of the feedback we got from customers was that the D70 has a small optional ML-L3 infrared remote trigger, a push-button you can use to trigger the camera.

ENGADGET: The Engadget Interview: Steve Heiner, General Manager, Digital SLR Systems, Nikon