单项选择题
A.before correcting for any deviation
B.after correcting for variation
C.after adjusting the fore-and-aft and athwartships permanent magnets
D.before the quadrantal correctors are placed on the compass
单项选择题 When a vessel changes course from one cardinal heading to another cardinal heading whilea djusting the compass，which action should be taken?（）
单项选择题 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?（）