单项选择题
The reference datum used in determining the heights of land features on most charts is（）.
A.mean sea level B.mean high water C.mean low water D.half-tide level
单项选择题 The purpose of the “set” hand on an aneroid barometer is to（）.
单项选择题 The error in the measurement of the altitude of a celestial body，caused by refraction，increases as the（）.
单项选择题 The boom stops on a pedestal crane prevent the boom from（）.
