单项选择题
When relieving the helm，the new helmsman should know the（）.
A.Course per magnetic steering compass B.Gyro error C.Variation D.Maximum rudder angle previously used
单项选择题 When crossing the magnetic equator the（）.
单项选择题 What is used to correct for both induced and permanent magnetism，and consequently must be readjusted with radical changes in latitude（）.
单项选择题 What is the basic principle of the magnetic compass（）.
