（）indicatesunstableweather.
A. Thepersenceofcirrusclouds
B. Thelow-hangingstratusclouds
C. Thealtocumulusclouds
D. Themassivecumulusclouds
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（）generallyindicatesthatagaleorstormisapproaching.
A. Fallingbarometricpressure
B. Risingbarometricpressure
C. Thecirculationsofwinds
D. Thechangeofwinddirection
（）isasilverycloudinthefrontoffeathersorfibresseenhighupagainstabluesky.
A. Cirrostratus
B. Cirrus
Cirrocmulus
D. Altostratus
（）isahighlayerofcloudinthefrontofsmallflakesorcauhflower,whiteincolourwithnodarkshadowsinbetween.
A. Cirrostratus
B. Cirrus
Cirrocumulus
D. Altostratus
Altostratusbelongsto（）.
A. Lowcl;ouds
B. Highclouds
C. Specialclouds
D. mediumclouds
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