单项选择题
The permanent magnetism of a vessel may change in strength due to（）.
A.a collision with another vessel B.being moored on a constant heading for a long period of time C.being struck by lightning D.All of the above
单项选择题 The permanent magnetism of a vessel may change in polarity due to（）.
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（）.
