单项选择题
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 True heading differs from magnetic heading by（）.
单项选择题 To center a compass bowl in its binnacle，you should have the ship on an even keel，heading north or south，and adjust the screws until（）.
单项选择题 The vertical component of the Earth’s magnetic field causes induced magnetism in vertical soft iron. This changes with latitude. What corrects for this coefficient of the deviation? （）