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 （）.
A.conspicuous object B.steep depression in the surrounding hills that resembles a cup C.domed structure useful for navigation D.calling-up-point used for traffic control
单项选择题 Navigational charts are （） frequent changes，the important one of which are promulgated by Admiralty Notices to Mariners．
单项选择题 My position has been obtained （） astronomical observation．
单项选择题 Magnetic information on a chart may be （）.
