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

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The purpose of the “set” hand on an aneroid barometer is to（）.

A.adjust the barometer
B.indicate any change in the reading of the barometer
C.provide a correction for height above sea level
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The error in the measurement of the altitude of a celestial body，caused by refraction，increases as the（）.

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