单项选择题
Isogonic lines are lines on a chart indicating （）.
A.points of equal variation B.points of zero variation C.the magnetic latitude D.magnetic dip
单项选择题 Lines on a chart which connect points of equal magnetic variation are called （）.
单项选择题 Many of the soundings shown on the chart are derived from （）.Undue reliance should not be placed upon them．
单项选择题 On this chart only the principle （） to navigation are shown．
