单项选择题
When a magnetic compass is not in use for a prolonged period of time it should（）.
A.be shielded from direct sunlight B.be locked into a constant heading C.have any air bubbles replaced with nitrogen D.have the compensating magnets removed
单项选择题 What is the name of the movable，radial guide line used to measure direction on a radar?（）
单项选择题 What is the basic principle of the magnetic compass?（）
单项选择题 What is an advantage of the magnetic compass aboard vessels?（）
