Certain courses are compulsory, others are optional.
某些课程是必修的，其他是选修的。
Optional extras include cooking classes at a top restaurant.
可选的额外之物包括一家高级餐馆的烹饪课程。
You took an optional course this semester, didn't you?
你这学期选了一门选修课，是吗？
Other bath accessories, such as a loofah sponge, or bath gels, or potpourri, are very optional.
洗澡需要的其他东西，比如丝瓜络、沐浴液、精油都是可选的。
Schools in our city provide a variety of optional classes to cater to students of different levels.
我们市里的学校提供各式各样的选修课，以满足不同水平的学生。
In the new curriculum, some courses are compulsory, while others are optional for middle school students.
在新课程中，一些课程是必修课，而另一些是中学生的选修课。
By setting Peking Opera as an optional course, students may have more freedom to decide whether to take it or not.
通过将京剧设置为选修课，学生们可以有更多的自由决定是否选修京剧。
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。
Others believe that going to classes should be optional for students.
另一些人认为学生应该可以选择去不去上课。
According to Professor Johnson, we don't have to read the book if we don't want to, as it is optional.
根据约翰逊教授的说法，我们不想读这本书，就不必读，因为它是选读的。
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。
These attributes are optional.
这些属性是可选的。
其他模板是可选的。
Validation is optional in DB2.
在DB 2中验证是可选的。
这是可以任选的功能部件。
Actions and predicate are optional.
动作和断言是可选的。
This is an optional parameter.
这是可选的参数。
It contains an optional description.
它含有一个可选描述。
See Figure 16 for optional Settings.
请参见图16以了解可选的设置。
The cycles are periodic and optional.
循环是阶段性和可选择的。
所有的代码都是可选的。
Fathers are not optional, though.
然而父亲可不是可有可无的。
Wearing the slippers is optional.
磨损的拖鞋可要可不要。
Otherwise, this parameter is optional.
否则，这个参数是可选的。
Complete the optional fields as needed.
根据需要填写可选字段。
其他标志可选。
新可选字段？
Optional: Garnish with lemon or lime slice.
可选：用柠檬片或青柠片装饰杯口。
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