单项选择题
To become an officer on board, you must obtain（）.
A.an officer's license B.an officer's promotion C.an officer's statement D.a seaman's book
单项选择题 In a weather report “GMT” stands for（）.
单项选择题 It is danger for vessels without the use of（）to approach the estuary in such poor visibility.
单项选择题 When visibility is 20 nautical miles, we say that visibility is（）.
