單項選擇題

A.magnetism from the earth's magnetic field
B.misalignment of the compass
C.magnetism within the vessel
D.a dirty compass housing

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A.deviation minus variation
B.variation plus compass course
C.combined variation and deviation
D.difference between true and magnetic heading

A.vessel's geographic position
B.vessel's heading
C.earth's magnetic field
D.influence of the magnetic materials of the vessel