单项选择题
A.An agonic line
B.The magnetic equator
C.A magnetic meridian
D.All of the above
单项选择题 The Flinders bar and the quadrantal spheres should be tested for permanent magnetism at what interval（）.
单项选择题 The Flinders bar on a magnetic compass compensates for the（）.
单项选择题 In more recent years，（）has been used by civilians in many new ways to determine positions，such as in automobile and boat navigation，hiking，emergency rescue，and precision agriculture and mining.