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（）.
A.Flinders bar B.Soft iron spheres C.Fore-and-aft permanent magnets in their trays D.Heeling magnet
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（）.
单项选择题 This induced magnetism（）the permanent magnetism already present in the ship.
