单项选择题 In shallow water，waves that are too steep to be stable，causing the crests to move forward faster than the rest of the wave，are called（）.

单项选择题 In most cases，the direction of the apparent wind lies between the bow and（）.

单项选择题 In modern fathometers the sonic or ultrasonic sound waves are produced electrically by means of a(n)（）.