单项选择题
Calibration Stations give special transmissions for the calibration of ship’s（）.
A.gyro-compass B.marine radar C.DF D.navigational satellite
单项选择题 To obtain accuracy in fixing by DF cross bearings，（）should be used.
单项选择题 To obtain accuracy in fixing by DF，（）and three stations should be used.
单项选择题 Vessels in port may use（）for receiving typhoon warnings during the typhoon season.
