单项选择题

The water level of the oceans rises and falls ______.

A．often
B．often in a year
C．twice
D．twice a day
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单项选择题
The water level of an ocean falls ______.
A．when there is a low tide
B．when the water is pulled away from the earth
C．when the moon disappears over the horizon
D．when water no longer moves
单项选择题
We have low tide ______.
A．when water reaches its highest level
B．when water stops
C．when water no longer moves
D．when water reaches its lowest level
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