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（）.

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（）.

A.Magnetic materials of the same polarity repel each other and those of opposite polarity attract
B.The Earth's magnetic lines of force are parallel to the surface of the Earth
C.Magnetic meridians connect points of equal magnetic variation
D.The compass needle（s）will，when properly compensated，lie parallel to the isogonic lines of the Earth

单项选择题Variation in a compass is caused by（）.

A.Worn gears in the compass housing
B.Magnetism from the earth's magnetic field
C.Magnetism within the vessel
D.Lack of oil in the compass bearings

单项选择题This induced magnetism（）the permanent magnetism already present in the ship.

A.will add to
B.will subtract from
C.may add to or subtract from
D.will not change

单项选择题The variation for most given areas undergoes（）change，the amount of which is also noted on charts.

A.an annual
B.a constant
C.a variable
D.an unstable