单项选择题
It is danger for vessels without the use of（）to approach the estuary in such poor visibility.
A.Loran C B.Direction Finder C.Gyro- compass D.Radar
单项选择题 When visibility is 20 nautical miles, we say that visibility is（）.
单项选择题 Our ship has not been to shipyard for a long time, the fouling on the ship’s hull greatly decreases her（）.
单项选择题 Radar is not only to obtain（）,but also to measure their distance away from you.
