单项选择题

A rotary current sets through all directions of the compass. The time it takes to complete one of these cycles is approximately（）.

A.3 hours
B.6 1/2 hours
C.12 1/2 hours
D.25 hours

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单项选择题

A light，feathery deposit of ice caused by the sublimation of water vapor directly into the crystalline form，on objects whose temperatures are below freezing，is called（）.

A.dew
B.frost
C.glaze
D.snow

单项选择题

A current will develop between areas of different density in ocean waters. If you face in the same direction the current is flowing，the water of（）.

A.high density will be on the left in the Northern Hemisphere
B.high density will be on the right in the Southern Hemisphere
C.low density will be on the left in the Northern Hemisphere
D.low density will be on the left in the Southern Hemisphere