Certain courses are compulsory; others are optional.
有些课程是必修的，其他是选修的。
This model comes with a number of optional extras(= things you can choose to have but which you will have to pay extra for) .
这一型号有一系列可供选择的附件，价格另计。
1.可自由选择的;随意的,任意的;非强制的
1.〈美〉选修科
1.something that is optional is available or possible if you want it, but you do not have to have it or do it
1.The new lock-in offer is part of a series of optional add-ons airlines have been rolling out in an effort to increase revenue.
这个全新的票价锁服务是其中一个航空公司为了增加盈利而推出的一系列附加服务选项。
2.The owner of each end point decides how much optional information to provide with the endpoint URL.
每个端点的所有者决定端点URL提供多少可选信息。
3.After a join hint is specified, the INNER keyword is no longer optional and must be explicitly stated for an INNER JOIN to be performed.
指定联接提示后，要执行INNERJOIN时，INNER关键字不再为可选，而必须显式说明。
4.In some smaller restaurants with fewer people dining, like the case of a couple of diners, the measure's adoption could be optional.
有一些小餐馆就餐人数比较少，比如一两个人、两三个人，可以有一些随意的情况。
5.In fact, for one-to-one associations that are optional on both end, the code generated by the wizard is all we need.
实际上，对于两端都可选的一对一关联，只需要向导生成的代码就够了。
6.A few days before a big social-science exam, the instructor informed us that he would hold an optional review session the next day.
在一场大型社会科学考试前几天，导师通知我们第二天他将会给我们上一节选修复习课。
7.This optional view appears at the bottom edge of a screen and contains segments that activate different modes in the application.
这个可选控件出现于应用程序的页面底部，用于切换程序的不同状态。
8.If you don't want the active rule to be reevaluated, you should include the rule's optional no-loop attribute and give it a value of true.
如果不希望这种情况发生，则应该包括rule的可选no-loop属性并将其赋值为true。
9.An optional step at this point may be to create a link aggregation device, commonly referred to as the LNAGG device.
（这里有一个可选步骤——创建一个链路聚合设备，通常称为LNAGG设备）。
10.Finally, while a dangling lure is optional, being hungry, Dr. Wilson says, at least metaphorically, "probably helps quite a bit. "
最后，虽然悬挂的诱饵可有可无，威尔逊说，但至少是从喻义上来说，饥饿状态“可能会很有帮助。”