单项选择题
A.Comparing the gyro azimuth of a celestial body with the computed azimuth of the body
B.Comparing the gyro heading with the magnetic compass heading
C.Determining from the chart the course made good between celestial fixes
D.It cannot be determined accurately at sea due to drift of unknown currents
单项选择题 The magnetism in the various structures of a ship changing as a result of cruising，vibration，or aging is termed（）.
单项选择题 The line which connects the points of zero magnetic dip is（）.
单项选择题 The Flinders bar and the quadrantal spheres should be tested for permanent magnetism at what interval（）.