单项选择题
A.magnetic equator
B.magnetic longitude reference line
C.points where there is no variation
D.points where there is no annual change in variation
单项选择题 Solid green arrows on the main body of a pilot chart indicate （）.
单项选择题 On the south and the east coasts of Block Island are circles with a dot in the center and labeled CUP. This is a （）.
单项选择题 Navigational charts are （） frequent changes，the important one of which are promulgated by Admiralty Notices to Mariners．