单项选择题
This induced magnetism（）the permanent magnetism already present in the ship.
A.will add to B.will subtract from C.may add to or subtract from D.will not change
单项选择题 The variation for most given areas undergoes（）change，the amount of which is also noted on charts.
单项选择题 The quadrantal spheres are used 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（）.
