You took an optional course this semester, didn't you?

你这学期选了一门选修课，是吗？

youdao

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

If you see humor as an optional form of entertainment, you're missing some of its biggest benefits: humor makes average-looking people look cute and uninteresting people seem entertaining.

如果你认为幽默只是一种可有可无的娱乐形式，那你就忽略了幽默最大的好处：幽默能让长相平平的人看起来更可爱，也能让呆板无趣的人看起来更有趣。

youdao

So I'm not suggesting that it's an optional problem set that you shouldn't do.

一个可做可不做的问题集,因为不做它你会后悔的。

麻省理工公开课 - 计算机科学及编程导论课程节选

"When it's an optional regulation, it's not that big of a deal, " Gradante said.

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The company just announced the Inspiron 15z, a 15-inch notebook with an optional touchscreen.

ENGADGET: Dell unveils Inspiron 15z Ultrabook with optional touchscreen, prices start at $750

This is especially useful when using the camera with an optional viewfinder (sold separately).

ENGADGET: Sony's Cyber-shot RX1 compact camera packs a full-frame sensor, 'fits in your palm' (hands-on)