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