单项选择题
A.only in fog
B.only between the hours of sunset and sunrise
C.only when entering and leaving port
D.at all times
单项选择题 A head-on situation at night is one in which you see（）.
单项选择题 One of the factors which affects the circulation of ocean currents is（）.
单项选择题 Vessels，（）support vessels，should not enter the safety zone of a deepwater port unless clearance has been obtained from the Vessel Traffic Supervisor.