When a magnetic compass is not in use for a prolonged period of time it should（）.

单项选择题What is the name of the movable，radial guide line used to measure direction on a radar?（）

A.Compass rose
B.Cursor
C.Plan position indicator
D.Variable range marker

单项选择题What is the basic principle of the magnetic compass?（）

A.Magnetic materials of the same polarity repel each other and those of opposite polarity attract
B.The Earth's magnetic lines of force are parallel to the surface of the Earth
C.Magnetic meridians connect points of equal magnetic variation
D.The compass needle(s) will，when properly compensated，lie parallel to the isogonic lines of the Earth

单项选择题What is an advantage of the magnetic compass aboard vessels?（）

A.Compass error is negligible at or near the earth's magnetic poles
B.It does not have to be checked as often
C.It is reliable due to it's essential simplicity
D.All points on the compass rose are readily visible

单项选择题What are the only magnetic compass correctors that correct for both permanent and induced effects of magnetism?（）

A.Quadrantal spheres
B.Heeling magnets
C.Athwartships magnets
D.Fore-and-aft magnets

单项选择题Variation is the angular measurement between（）.

A.compass north and magnetic north
B.compass north and true north
C.magnetic meridian and the geographic meridian
D.your vessel's heading and the magnetic meridian