单项选择题 Before（）, a station shall take precautions to ensure that its emission will not interfere with transmission already in progress.

单项选择题 Two vessels（）while they re sailing in Huangpu River.

单项选择题 （）all ships in a large sea area being alerted, an “area call” will normally be transmitted so that only those ships in the vicinity of the distress incident are alerted.