单项选择题
Cirrus clouds followed by cirrostratus then altostratus，stratus，and occasionally nimbostratus indicate the approach of a(n)（）.
A.cold front B.warm front C.tropical front D.occluded front
单项选择题 Brief，violent showers frequently accompanied by thunder and lightning are usually associated with（）.
单项选择题 As a cold front passes an observer，pressure（）.
单项选择题 An occluded front is usually caused by a（）.
