（）are mainly caused by the gravitational pull of the moon.

单项选择题（）generally indicates that a gale or storm is approaching.

A.Falling barometric pressure
B.Rising barometric pressure
C.The circulations of winds
D.The change of wind direction

单项选择题（）indicates unstable weather.

A.The persence of cirrus clouds
B.The low-hanging stratus clouds
C.The altocumulus clouds
D.The massive cumulus clouds

单项选择题（）may be a warning for an approaching storm.

A.The persence of cirrus clouds
B.The low-hanging stratus clouds
C.The altocumulus clouds
D.The massive cumulus clouds

单项选择题Nimbus means（）.

A.rain
B.high
C.layer
D.cirrus

单项选择题Alto stands for（）.

A.rain
B.high
C.layer
D.cirrus