单项选择题 A chart position enclosed by a semi-circle is a(n) （）.

单项选择题 （）is a term denoting the determination of a ship’s position by observations of Celestial or terrestrial objects，or by a combination of both．

单项选择题 The changes in the channel’s sands and buoys on this coast are （） this chart can not be considered as a safe guide of the channel．