| 例句 |
adjective 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。 Certain classes are compulsory, others are optional. These features can be offered as an optional part of the package. This model comes with a number of optional extras. 这一型号有一系列可供选择的附件，价格另计。 |