单项选择题
The numeral in the center of a wind rose circle on a pilot chart indicates the （）.
A.total number of observations B.average wind force on the Beaufort scale C.average wind force in knots D.percentage of calms
单项选择题 The maximum difference between mean time and apparent time is （）.
单项选择题 The description Racon beside an illustration on a chart would mean a （）.
单项选择题 The compass rose on a nautical chart indicates both variation and （）.
