单项选择题
A.They are not subject to permanent magnetism; no check is necessary
B.Semiannually
C.Annually
D.Every five years
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.
单项选择题 By convention，the north seeking ends of a compass’ magnets are colored（）.