单项选择题
A generally circular low pressure area is called a(n)（）.
A.cyclone B.anticyclone C.cold front D.occluded front
单项选择题 A frontal thunderstorm is caused by（）.
单项选择题 A cyclone in its final stage of development is called a(n)（）.
单项选择题 A cloud sequence of cirrus，cirrostratus，and altostratus clouds followed by rain usually signifies the approach of a(n)（）.
