单项选择题
When crossing the magnetic equator the（）.
A.Flinders bar should be inverted B.Heeling magnet should be inverted C.The quadrantal spheres should be rotated 180° D.Flinders bar should be moved to the opposite side of the binnacle
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（）.
单项选择题 Variation in a compass is caused by（）.
