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
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
单项选择题
Shining down out of the blue sky, ______.
A．their pale skins were burnt by the sun
B．they were burnt by the fierce sun
C．the sun burnt their tender skins
D．the sun burning their pale skins
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