单项选择题 Clouds that form as small white flakes or scaly globular masses covering either small or large portions of the sky are（）.

单项选择题 Clouds are classified according to their（）.

单项选择题 Clouds appearing in the following order: cirrus，cirrostratus，altostratus，stratus，and nimbostratus usually indicate the approach of a(n)（）.