单项选择题
Clouds appearing in the following order: cirrus，cirrostratus，altostratus，stratus，and nimbostratus usually indicate the approach of a(n)（）.
A.warm front B.occluded front C.medium front D.cold front
单项选择题 Cloud formations are minimal when the（）.
单项选择题 Cirrus clouds followed by cirrostratus then altostratus，stratus，and occasionally nimbostratus indicate the approach of a(n)（）.
单项选择题 Brief，violent showers frequently accompanied by thunder and lightning are usually associated with（）.
