单项选择题 A type of precipitation that occurs only in thunderstorms with strong convection currents that convey raindrops above and below the freezing level is known as（）.

单项选择题 A twin-screw vessel with a single rudder is making headway. The engines are full speed ahead. There is no wind or current. Which statement is FALSE? （）

单项选择题 A tug may be in danger of tripping when（）.