单项选择题
A cloud sequence of cirrus，cirrostratus，and altostratus clouds followed by rain usually signifies the approach of a(n)（）.
A.occluded front B.stationary front C.warm front D.cold front
单项选择题 A cloud of marked vertical development (often anvil-shaped) would be classified as（）.
单项选择题 A boundary between two air masses is a(n)（）.
单项选择题 （）causing strong winds and rough sea north west Bay of Biscay.
