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1. The policeman used his signal whistle to alert the crowd.
警察用他的信号哨子来警告人群。
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2. The firefighters used their signal whistles to coordinate their efforts.
消防员使用他们的信号哨子来协调他们的努力。
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3. The pilot used his signal whistle to communicate with the ground crew.
飞行员使用他的信号哨子与地面机组进行通信。