单项选择题
What is an advantage of the magnetic compass aboard vessels?（）
A.Compass error is negligible at or near the earth's magnetic poles B.It does not have to be checked as often C.It is reliable due to it's essential simplicity D.All points on the compass rose are readily visible
单项选择题 What are the only magnetic compass correctors that correct for both permanent and induced effects of magnetism?（）
单项选择题 Variation is the angular measurement between（）.
单项选择题 Variation is a compass error that you（）.
