单项选择题
A.use the magnets one at a time，putting one in one side and then one on the opposite side，one step higher
B.use the magnets in pairs，starting at the top，with trays at the highest point of travel
C.use the magnets in pairs，from the bottom up，with the trays at the lowest point of travel
D.fill all the trays with magnets，then remove them one-by-one until the deviation is removed
单项选择题 When adjusting a magnetic compass for error，a deviation table should be made（）.
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（）.