单项选择题 As a ship moves through the water，it drags with it a body of water called the wake. The ratio of the wake speed to the ship’s speed is called（）.

单项选择题 Apparent wind speed blowing across a vessel under tow can be measured by a(n)（）.

单项选择题 A vessel，which is unable to maneuver due to some exceptional circumstance，shall exhibit （）.