单项选择题 A swift current occurring in a narrow passage connecting two large bodies of water，which is produced by the continuously changing difference in height of tide at the two ends of the passage，is called a（）.

单项选择题 A survival craft being used to pick up a person who has fallen overboard from a ship should approach the person（）.

单项选择题 A sufficient amount of chain must be veered when anchoring a vessel to ensure（）.