单项选择题
A.size
B.moisture content
C.altitude and how they were formed
D.location in a front
单项选择题 Clouds appearing in the following order: cirrus，cirrostratus，altostratus，stratus，and nimbostratus usually indicate the approach of a(n)（）.
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)（）.