单项选择题
Before switching on gyro-compass，you should make sure that the power supply on board is（）.
A.high or low B.AC or DC C.strong or weak D.on or off
单项选择题 Calibration Stations give special transmissions for the calibration of ship’s（）.
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.
