形容词
中文翻译：警惕的
同义词：
['vigilant', 'watchful', 'wary']
例句：
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1. The policeman was alert to the danger of the situation.
警察对形势的危险保持警惕。
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2. She is always alert to her surroundings.
她总是对她周围的环境保持警惕。
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3. The driver was alert when he saw the oncoming car.
司机在看到迎面而来的车时保持警惕。