单项选择题
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
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
A.nonmagnetic B.weatherproof C.corrosion resistant D.capable of being permanently affixed to the vessel