单项选择题
A.being moored for a long time on one heading
B.being struck by lighting
C.steaming from the north magnetic hemisphere to the south magnetic hemisphere
D.loading a homogenous magnetic cargo such as steel plate，iron bars，etc
单项选择题 The MOST important feature of the material used for making the binnacle of a standard magnetic compass is that it is（）.
单项选择题 The magnetic compass operates on the principle that（）.
单项选择题 The magnetic compass magnets are acted on by the horizontal component of the Earth’s total magnetic force. This magnetic force is GREATEST at the（）.