When adjusting a magnetic compass using the fore-and-aft permanent magnets，you should（）.
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
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