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

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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

单项选择题

The quadrantal spheres are used to（）.

A.Remove deviation on the intercardinal headings
B.Remove deviation on the cardinal compass headings
C.Remove heeling error
D.Compensate for induced magnetism in vertical soft iron