optional

adj.

可选择的；随意的，任意的；非强制的；选修科目

optional course选修课，选修课程

optional parameter任选参数，可选参数

optional exercise自选动作

optional equipment【计】附加设备；任选设备

optional item可选择项目

1. The insurance cover is optional.
   保险范围可以随意选择。
2. It is optional with you.
   那是你的自由。
3. The goods are optional, but only one.
   这些物品是可以任选的, 但只能选一个。
4. A 4-speed automatic transmission is optional.
   四段自动变速器是可自由选择的。

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...Anonymous Credentials with Optional Anonymity ...
JADE: adaptive differential evolution with optional external archive
An Ecient System for Non-transferable Anonymous Credentials with Optional Anonymity Revocation
Evidence supports optional use of RAI for papillary thyroid cancer
Specific language impairment as a period of extended optional infinitive.
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The English Education System at National Institute of Technology, Kisarazu College : The Introduction of "Optional or Voluntary Acti...
Application of a New Teaching Method in Optional Courses
Evaluation of optional fee structures for solid waste management in China