Variation in a compass is caused by（）.……

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

单项选择题The MOST important feature of the material used for making the binnacle of a standard magnetic compass is that it is（）.

A.nonmagnetic
B.weatherproof
C.corrosion resistant
D.capable of being permanently affixed to the vessel

单项选择题The most accurate method of determining gyrocompass error while underway is by（）.

A.Comparing the gyro azimuth of a celestial body with the computed azimuth of the body
B.Comparing the gyro heading with the magnetic compass heading
C.Determining from the chart the course made good between celestial fixes
D.It cannot be determined accurately at sea due to drift of unknown currents