| 1. | Formal dress is optional . |
| 是否穿礼服自便。
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| 2. | optional subjects at school |
| 学校的选修课
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| 3. | Mnemonics are not optional . |
| 访问键不是可有可无的，而是必须要有的。
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| 4. | The current practice of allowing candidates to register for an exam without purchasing the curriculum sends the wrong message that studying the curriculum is optional . |
| 目前的做法允许候选人参加考试而无需购买教材，这传达了教材可有可无的错误信息。
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