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

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

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