单项选择题
What are the only magnetic compass correctors that correct for both permanent and induced effects of magnetism?（）
A.Quadrantal spheres B.Heeling magnets C.Athwartships magnets D.Fore-and-aft magnets
单项选择题 Variation is the angular measurement between（）.
单项选择题 Variation is a compass error that you（）.
单项选择题 Variation in a compass is caused by（）.
