单项选择题
The variation for most given areas undergoes（）change，the amount of which is also noted on charts.
A.an annual B.a constant C.a variable D.an unstable
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（）.
单项选择题 The most accurate method of determining gyrocompass error while underway is by（）.
