单项选择题
The error in the measurement of the altitude of a celestial body，caused by refraction，increases as the（）.
A.horizontal parallax decreases B.observer's height above sea level increases C.humidity of the atmosphere decreases D.altitude of the body decreases
单项选择题 The boom stops on a pedestal crane prevent the boom from（）.
单项选择题 The average height of the surface of the sea for all stages of the tide over a 19 year period is called（）.
单项选择题 The 3-cm radar as compared to a 10-cm radar with similar specifications will（）.
