单项选择题
Echo sounder sends a（） (无线电信号)from the bottom of the ship to the seabed.
A. radio signal B. radio wave C. signal D. signal radio
单项选择题 Radar is of great help to ship in poor（）(能见度).
单项选择题 Navigators use navigational aids, such as（）（罗经）.
单项选择题 （）（电罗经）can tell us true north.
