A cloud sequence of cirrus，cirrostratus，and altostratus clouds followed by rain usually signifies the approach of a(n)（）.

单项选择题A cloud of marked vertical development (often anvil-shaped) would be classified as（）.

A.cirrus
B.cirrocumulus
C.altocumulus
D.cumulonimbus

单项选择题A boundary between two air masses is a(n)（）.

A.lapse rate
B.isobar
C.front
D.continent

单项选择题（）causing strong winds and rough sea north west Bay of Biscay.

A.Strong low pressure
B.Strong high pressure
C.Steep pressure gradient
D.Steep gradient pressure