单项选择题
A.in high latitudes B.near the equator C.on north or south headings D.on intercardinal headings
A.induced magnetism in the vessel's horizontal soft iron B.change in the magnetic field when the vessel inclines from vertical C.permanent magnetism of the vessel D.magnetic fields caused by electrical currents in the vicinity
A.the vessel's heading B.sea conditions C.seasons D.the vessel's position