短语
中文翻译：警惕
同义词：
['be on the lookout', 'be vigilant', 'be wary']
例句：
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1.He was on his guard when he entered the room.
当他进入房间时，他保持警惕。
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2. The police were on their guard for any suspicious activity.
警察对任何可疑活动都保持警惕。
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3. The thief was caught by the police on his guard.
小偷被警方在防备中抓获。