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飞行员必须随时保持警惕。
A pilot must remain vigilant at all times.
《牛津词典》
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在一个完全陌生的环境里，你必须随时保持警惕。
You must keep alert when you are in a completely unknown environment.
youdao
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在一个完全陌生的环境里，你必须随时保持警惕。
You must keep alert when you are in a completely unknown environment.
youdao