Somewhere along the line, holiday tipping went from an optional thank-you for a year of services to a Mafia-style protection racket.

一路走来，节日小费从对一年的服务表示感谢的选择变成了黑社会性质的勒索保护费。

youdao

There is a tour guide throughout the trip who will organize optional dinners and sightseeing.

有一位全程导游，可安排非强制性的进餐和观光。

《柯林斯英汉双解大词典》

Sex education is a sensitive area for some parents, and thus it should remain optional.

性教育对一些家长来说是个敏感的话题，因此应该还是非强制性的。

《柯林斯英汉双解大词典》

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